Structures and fabrication methods for solar cells and modules

ABSTRACT

Solar module structures and methods for assembling solar module structures. The solar module structures may be used in solar glass applications, building facade applications, rooftop installation applications as well as for centralized solar electricity generation.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/188,303 filed Jul. 21, 2011 which is a continuation of U.S. patentapplication Ser. No. 11/868,494 filed Oct. 6, 2007 which claims thebenefit of U.S. Prov. Pat. Nos. 60/828,678 filed on Oct. 9, 2006 and60/886,303 filed on Jan. 24, 2007, all of which are hereby incorporatedby reference in their entirety.

FIELD

This disclosure relates in general to the field of photovoltaics andsolar cells, and more particularly to solar module structures andassembly methods. Even more particularly, the presently disclosedsubject matter relates to solar module structures and assembly methodsfor pyramidal three-dimensional (3-D) thin-film solar cells (TFSCs).

DESCRIPTION OF THE RELATED ART

Renewable, high-efficiency, and cost-effective sources of energy arebecoming a growing need on a global scale. Increasingly expensive,unreliable, and environmentally-risky fossil fuels and a rising globaldemand for energy, including electricity, have created the need foralternate, secure, clean, widely available, cost-effective,environmentally-friendly, and renewable forms of energy. Solarphotovoltaic (PV) electricity generation using solar cells is uniquelysuited to meet the needs of residential, commercial, industrial, andcentralized utility applications. Key attributes that make solar energyattractive are the abundant, worldwide, point-of-use supply of sunlight,environmental friendliness, scalability (from milliwatts to megawatts),secure point-of-use generation of solar electricity, and excellentdistributed energy economics. The sun provides more energy to the earthin one hour than the annual energy consumption of the entire world. Muchof the earth's surface receives a significant amount of annual sun-hourswhich may be effectively harnessed for clean and secure electricitygeneration. A key driver for this market pull is a rising publicawareness of environmentally-benign technologies. However, due torelatively low solar cell efficiencies (e.g., less than 12% for mostthin-film technologies and roughly 12% to 18% for most crystallinesilicon solar cell technologies), high costs of raw materials (e.g.,silicon for crystalline silicon wafer solar cells) and manufacturingprocesses, limitations on cost-effective and efficient electricalstorage, and a general lack of infrastructure to support solar cellproliferation, to date there has been limited use of this energysolution (currently, electricity generation by solar photovoltaicsaccounts for less than 0.1% of total worldwide electricity generation).

For commercial applications, cost of energy to the end-user (e.g., incents/kWh for electricity) should be sufficiently low and comparable toor even better than that from utility grids using conventionalelectricity generation sources. The solar photovoltaic electricitygeneration, which currently accounts for less than 0.1% of the globalelectricity generation, may be substantially expanded if it achievescost parity with conventional grid electricity. As the costs of solarcells and modules (typically expressed as $/W_(p)) are reduced,grid-tied solar photovoltaic applications are gaining acceptance at anaccelerated pace, making them an attractive option for significantproliferation in electricity generation.

In the price-sensitive solar cell market, two principal technologyoptions exist. On the one hand, crystalline silicon (c-Si) wafers mayserve as the basis for solar cell formation (currently accounting formore than 90% of the solar PV market). On the other hand, thin-film(amorphous and polycrystalline) technologies using silicon and othersemiconductor absorber materials (such as amorphous silicon, CdTe, orCIGS) may offer significant cost advantages compared to crystallinesilicon wafer-based solar cells. These different approaches are atopposite ends of the price-performance scale. Crystalline silicon wafersoffer higher performance, but at higher costs (due to the relativelyhigh cost of starting monocrystalline and multicrystalline siliconwafers). Thin-film technologies may offer lower manufacturing costs, buttypically at lower performance levels (i.e., lower efficiencies). Forboth approaches, the price-per-watt typically increases as cellefficiencies rise (due to higher material and/or manufacturing costs).

Due to a rapid annual growth rate of more than 40% during the past tenyears and the concurrent demands for silicon material by bothsemiconductor microelectronics and solar PV industries, the solar PVindustry has been experiencing a shortage of polysilicon feedstocksupply. The polysilicon feedstock shortage has significantly constrainedthe solar PV industry growth, particularly during the past severalyears. In fact, the solar cell industry currently consumes over half ofthe worldwide production of high-purity polysilicon feedstock. Withinthe last few years, the contract price of polysilicon has increased fromroughly $30/kg to roughly $85/kg, with spot prices exceeding $250/kg.This has led to large increases in the price of monocrystalline andmulticrystalline silicon wafers, which now account for roughly half ofthe total solar module manufacturing cost.

The trend in the mainstream crystalline silicon (c-Si) wafer solar cellindustry has been to scale down wafer thicknesses to below 200 microns(in order to reduce the amount of silicon material in grams used perwatt of solar cell rated peak power). For example, monocrystallinesilicon wafer solar cells are projected to scale down to a thickness ofroughly 120 microns by 2012, from a current wafer thickness of roughly200 microns. Multicrystalline silicon wafer solar cells are projected toscale down to a thickness of roughly 180 microns by 2012, from a currentaverage wafer thickness of roughly 260 microns. This wafer thicknessreduction, however, presents additional challenges related to mechanicalrigidity, manufacturing yield, and solar cell efficiency. Despite itshigh cost, crystalline silicon (c-Si) technology still dominates thesolar cell market, mainly due to higher efficiencies and synergies withthe established microelectronics industry and supply chain. Currently,c-Si accounts for slightly over 90% of the solar cell market (95% whenribbon silicon is included).

Historically, crystalline silicon solar cells have achieved a 20% costreduction for each doubling of cumulative global cell production(measured in megawatts or MW_(p) and gigawatts or GW_(p)). It isprojected that through innovative cost reduction and efficiencyenhancement methods, the cost of electricity derived from grid-connectedrooftop solar photovoltaic modules may become comparable to the cost ofelectricity purchased from the utility grid in five to ten years. A 2005survey of the commercially available monocrystalline silicon andmulticrystalline silicon solar modules reports the solar moduleefficiencies then in the range of 9.1% to 16.1%, with a medianefficiency value of about 12.5%. Commercial crystalline silicon modulesusually show a rapid initial efficiency degradation of 1% to 3%(relative) due to various effects, including photodegradation effects inwafered solar cells (e.g., wafer minority carrier lifetime degradation).Monocrystalline silicon wafer solar cell efficiencies are projected toincrease to roughly 20.5% by 2012, from a current efficiency of roughly16.5% (leading-edge commercially available monocrystalline silicon solarcell and solar module efficiencies are currently about 21.5% and 18%,respectively). Multicrystalline silicon wafer solar cell efficienciesare projected to increase to roughly 18% by 2012, from a currentefficiency level of roughly 15.5%.

State-of-the-art crystalline silicon solar cell manufacturing currentlyuses about 10 grams of high-purity polysilicon feedstock per peak watt(g/W_(p)), resulting in a polysilicon feedstock material cost of about$0.85/W_(p) (assuming a polysilicon price of $85/kg). Over the next fiveyears, the projected trends of solar cell wafer thickness reduction(e.g., to less than 200 micron wafers) and a long-term assumed price ofabout $20/kg for solar-grade polysilicon may reduce the polysiliconfeedstock cost (in g/W_(p)) by about a factor of four to eight to about$0.10/W_(p) to $0.20/W_(p). Thus, any competing solar cell technologiesshould benchmark their manufacturing cost goals against this reduced rawmaterial cost number. For a given cell efficiency, silicon waferthickness reduction presents a prime opportunity for solar cell costreduction by reducing the amount of polysilicon feedstock consumed perwatt of peak solar power.

The cost associated with wire saws, amounting to about $0.25/W_(p) forcurrent silicon solar cells provides another wafer-related costcomponent for silicon wafer solar cells. Innovative and cost-effectivetechnologies that eliminate the kerf losses associated with sawing andslicing should further facilitate silicon solar cell cost reductions. Itis projected that the wafer-based crystalline silicon solar modulemanufacturing cost (which is currently on the order of $2.10 per watt tomore than $2.70 per watt) may be reduced to the range of roughly$1.50/W_(p) to $1.80/W_(p) by the year 2012, in part due to wafer sawingkerf loss reduction to roughly 130 microns by 2012 from the currentvalue of roughly 200 microns. The overall cost reductions forwafer-based crystalline silicon solar cells may come from varioussources including: lower cost polysilicon feedstock, thinner wafers,higher cell-level efficiencies, reduced wafer sawing kerf losses, andincreased economy of scale or manufacturing volume.

State-of-the-art silicon wafer solar cell fabrication facilities (“solarfabs”) typically produce 125 mm×125 mm up to 156 mm×156 mm solar cellstoday. The trend in crystalline silicon wafer solar cells is towardthinner and larger wafers. The monocrystalline and cast (as well asribbon) multicrystalline silicon solar cell wafer thicknesses inleading-edge solar cells used for power generation modules are projectedto be reduced to around 150 and 200 microns, respectively, by around2009-2010. Any cost-effective, high-efficiency, innovative silicon solarcell technology which enables a substantial reduction of the siliconmaterial consumption (e.g., wafer or film thickness) per W_(p) of cellpower compared to the above-mentioned current and projected 2009-2010numbers may offer significant promise as a viable commercial solar celltechnology for solar photovoltaic applications (e.g., residential,commercial, and industrial rooftop as well as large-scale centralizedutilities electrical power generation applications).

Higher solar cell efficiencies have favorable effects on the entiresolar cell value chain and levelized cost of energy (LCOE in $/kWh) dueto reduced material consumption and cost as well as reducedbalance-of-system (BOS) costs (e.g., area-related solar moduleinstallation and inverter costs). The current mainstream commercialcrystalline solar cells provide efficiencies on the order of 14% to 17%.It is expected that the projected crystalline silicon solar cellefficiencies in commercial solar cells may approach around 19% and 17%for monocrystalline and multicrystalline silicon solar cells,respectively, by the year 2009. A key area for new solar cell businessopportunities is development of innovative cell structures andsimplified process flows which may drive efficiencies up while loweringoverall solar cell and module manufacturing costs. For alternative(e.g., thin-film PV) approaches to succeed over the mainstreamwafer-based crystalline silicon solar cell technologies, they shouldprovide higher efficiencies at even lower manufacturing costs comparedto the projected efficiency and cost numbers for the mainstreamwafer-based crystalline silicon solar cells when the new technology isfully commercialized.

Economy-of-scale fab cost reduction associated with high-volume solarfab capacities is a key factor impacting LCOE. The state-of-the-arthigh-volume solar photovoltaic fabs have annual production capacities onthe order of or in excess of 50 MW_(p) to 100 MW_(p) (MW_(p=)1 millionW_(p)). High-volume solar photovoltaic fab capacities are expected toincrease substantially to annual production rates of several hundredMW_(p) or even approaching 1 GW_(p) (GW_(p=)1 billion W_(p)) in thecoming decade. While very-high-volume solar fabs in the range of 100MW_(p) to 1 GW_(p) should facilitate longer term cost reductions(including LCOE) through high-volume manufacturing economies of scale,the relatively high initial fab investment costs, which may easilyexceed $100M, may impose certain limits on solar photovoltaic fabconstruction options. Ideally, the preference may be to developinnovative crystalline silicon solar cell designs and simplifiedmanufacturing processes which facilitate substantial manufacturing costreductions in solar cells and modules even in smaller-scale (and lesscapital intensive) fabs with modest production volumes (e.g., annualproduction volumes in the range of 5 MW_(p) to 50 MW_(p)). This type oftechnology would allow for modest-volume solar photovoltaic fabs withmodest fab setup and operation costs. Reduced fab setup and operationcosts would further facilitate global proliferation of cost-effectivesolar modules, enabling construction of a multitude of very affordablemodest-volume fabs (in contrast to having to set up very expensivehigh-volume fabs in order to achieve sufficient economy of scale formanufacturing cost reduction). Of course, an innovative solar celltechnology that meets the above-mentioned criteria for cost-effective,modest-volume fabs (i.e., meeting the LCOE roadmap requirements even atmodest production volumes in low-cost fabs set up for simplified solarcell processing), may also be applicable to very-high-volume (e.g.,greater than 100 MW_(p)) solar fabs. Such solar photovoltaic fabs cantake further advantage of the economies of scale associated withincreased volume.

Thin-film solar cell (TFSC) technologies (e.g., amorphous silicon, CdTe,and CIGS) require little absorber material (usually much less than 10microns in thickness) to absorb typical standard “Air Mass 1.5” (AM-1.5)solar illumination due to absorption bands that are well matched to thesolar spectrum. The TFSC absorber material may be deposited oninexpensive substrates such as glass or flexible metallic ornon-metallic substrates. TFSCs typically offer low cost, reduced moduleweight, reduced materials consumption, and a capability for usingflexible substrates, but are usually much lower in efficiency (e.g.,usually 5% to 12%). In the case of prior art thin crystalline siliconfilms, there are a number of major problems and challenges with the useof flat silicon films (such as epitaxially growth silicon films withthicknesses below 50 microns) for low-cost, high-performance solarcells. These include: relatively low solar module efficiencies(typically 7% to 12%), field degradation of module efficiencies, scarceand expensive absorber materials (e.g., In and Se for CIGS and Te forCdTe), limited validation of system field reliability, and adverseenvironmental impact of non-silicon technologies such as CIS/CIGS andCdTe.

Prior art FIG. 1 shows process flow 10 for fabricating c-Si TFSCs usingplanar silicon thin-film absorber layers produced by epitaxial silicon.This prior art TFSC fabrication process flow uses several shadow maskprocess steps to form the cell structure. The cell absorber is simply athin planar film of c-Si formed by silicon epitaxial growth processing.The cell uses frontside silicon texturing to improve light trapping anda detached rear aluminum mirror to improve the cell efficiency. Step 12starts with single-crystal p³⁰ CZ silicon. Step 14 involveselectrochemical HF etching of silicon to form 2-layer porous siliconcomprising a 1 micron top layer with 20% porosity and a 200 nanometerrear layer with greater than 50% porosity. Step 16 involves a hydrogen(H₂) anneal at 1100° C. for 30 minutes. Step 18 involves epitaxialsilicon growth at 1100° C. using trichlorosilane or SiHCl₃ (depositionrate of 1 micron per minute), forming 2 microns of p⁺-Si and 30 micronsof p-Si. Step 20 involves frontside surface texturing by wet KOH etchingto form upright surface pyramids. Step 22 involves the first shadow maskprocess, with LPCVD silicon nitride (SiN_(x)) deposition through ashadow mask to define emitter diffusion windows. Step 24 involves solidsource phosphorus diffusion at 830° C. (to achieve 80 Ω/square for then⁺ doped junction). Step 26 involves the second shadow mask process,with frontside metallization (titanium/Pd/silver grid) by evaporationthrough shadow mask. Step 28 involves emitter surface passivation byhydrogenated PVD or PECVD SiN_(x). Step 30 involves contact frontsidebusbar by a conductive adhesive. Step 32 involves gluing the cellfrontside to MgF₂-coated glass using clear glue. Step 34 involvesseparating the cell from silicon wafer by mechanical stress. Step 36involves the third shadow mask process, with backside aluminummetallization using evaporation through shadow mask. Finally, step 38involves attaching an aluminum reflector at 200 micron spacing from thecell backside.

Prior art FIG. 2 shows another process flow method 40 for fabrication ofsolar cells on silicon wafers with self-aligned selective emitter andmetallization. This prior art process uses laser processing to patternthe top cell dielectric layer while melting the underlying silicon toform the heavily-doped n⁺⁺ emitter contact diffusion regions (afterformation of the lightly diffused selective emitter regions by rapidthermal annealing). Step 42 starts with single-crystal p-type silicon.Step 44 involves saw damage removal etch and anisotropic texturing etchin dilute NaOH at 90° C. Step 46 involves spin-on application and dryingof phosphorus diffusion source. Step 48 involves rapid thermal annealingto form lightly diffused emitter (80 to 200 Ω/square). Step 50 involvesapplication of backside metal contact by vacuum evaporation or screenprinting of aluminum or silver/aluminum alloy, followed by drying. Step52 involves backside metal sintering/firing (e.g., at 820° C. inoxygen/nitrogen) for a screen-printed contact (fires the metal pastewhile oxidizing the dielectric to raise its resistance to the metalplating solution). Step 54 involves laser processing to pattern the topdielectric layer while melting the underlying silicon to form the n⁺⁺contact diffusion region. Step 56 involves dilute HF etch to preparemetal plating surface. Step 58 involves electroless nickel plating at90° C. for five minutes. Step 60 involves nickel sintering at 350° C. to450° C. (in nitrogen, argon, or forming gas). Step 62 involves anadditional 2 minutes of nickel plating followed by long electrolesscopper plating to form thick high-conductivity copper film. Step 64involves flash immersion silver (silver) deposition on copper surface.Finally, step 66 involves edged junction isolation (e.g., using lasergrooving, edge cleavage, or plasma etching).

With regard to the prior art crystalline silicon (c-Si) thin-film solarcell (TFSC) technology, there are difficulties associated withsufficient surface texturing of the thin silicon film to reduce surfacereflectance losses, while reducing the crystalline silicon filmthickness. This places a limit on the minimum flat (co-planar)monocrystalline silicon thickness due to production yield and cellperformance (efficiency) considerations. In the case of a flat orco-planar film, it is essential to use surface texturing since thereflectance of an untextured crystalline silicon film is quite excessive(can be greater than 30%) and results in substantial optical reflectionlosses and degradation of the external quantum efficiency. Thus,reduction of reflectance-induced photon losses in co-planar epitaxialsilicon films requires effective surface texturing which itself places alimit on the minimum epitaxial silicon layer thickness. Depending on thefilm surface texturing requirements and processes, the minimumcrystalline silicon layer thickness may be on the order of at least 10microns (so that the texturing process does not break through anyportions of the crystalline silicon layer).

In addition, substantially reduced mean optical path lengths in thinplanar crystalline silicon films result in reduced photon absorption,particularly for photons with energies near the infrared bandgap ofsilicon (800 to 1100 nanometers), resulting in reduced solar cellquantum efficiency (reduced short-circuit current or J_(sc)). Thisresults in serious degradation of the solar cell efficiency due toreduced cell quantum efficiency and reduced J_(sc). For instance, in aco-planar (flat) crystalline silicon absorber layer with thickness of 20microns, a solar light beam impacting the cell at a near-normal anglewould have an effective path length equal to the film thickness, far tooshort for the solar radiation photons with energies near the infraredbandgap of silicon (i.e., with wavelengths of roughly 800 to 1100nanometers) to be absorbed effectively in the silicon thin film. Infact, a reduction of the active cell silicon thickness to below roughly50 microns results in appreciable reduction of J_(sc) and the resultingsolar cell efficiency, with this degradation effect rapidly acceleratingwhen the silicon film thickness is reduced below roughly 20 microns.Thus, a co-planar thin crystalline silicon film may also requireeffective light trapping using both top surface texturing and rearsurface back reflection of the light exiting the back surface of thecrystalline silicon film in order to create effective optical pathlengths equal to a large multiple of the crystalline silicon filmthickness.

The prior art technologies using this approach mostly use either backreflection through internal reflection of the light at the crystallinesilicon film/silicon substrate, or reflection from a blanket backsidecontact (such as a back surface field aluminum contact/mirror). The backreflectance provided by these techniques may not be great (e.g., roughly70% effective near-IR rear reflectance), constraining the performancegain that would have otherwise been achieved by an optimal backreflector. The problem with this approach is that the primary incidentbeam always passes the crystalline silicon film only once. Anysubsequent second passes of the primary incident beam photons aredependent on the back surface reflection.

There is also the problem of lack of rigidity and mechanical support ofthe thin film during cell and module processing steps. This problemrelates to the mechanical strength of a large-area (e.g., 200 mm×200 mm)thin silicon film. It is well known that reducing the large-areacrystalline silicon wafer thickness to below 100 microns results in asubstantial loss of cell substrate mechanical strength/rigidity, andsuch thin wafers tend to be flexible and very difficult to handlewithout breakage during cell fabrication process flow.

Large-area, co-planar (flat) crystalline silicon films thinner than, forinstance, 50 microns must be properly mounted and supported on acost-effective support or handle substrate in order to achieveacceptable yield for solar cell and module manufacturing. One approachis to grow and retain the thin epitaxial film on a relatively low-cost(e.g., metallurgical-grade) silicon substrate (over which the epitaxiallayer is grown); however, this approach suffers from some inherentproblems constraining the ultimate solar cell efficiency. Anotherapproach is to release or lift off the epitaxial silicon film from its(reusable) parent silicon substrate and subsequently place it on acheaper non-silicon support or handle substrate to provide mechanicalstrength through the solar cell process flow. This approach may sufferfrom any thermal coefficient of expansion (TCE) mismatch between thesupport/handle substrate and silicon film during any high-temperatureoxidation and anneal processes, as well as potential contamination ofthe thin epitaxial silicon film from the non-silicon support substrate(both creating possible manufacturing yield and performance/efficiencydegradation problems).

The cost of the monocrystalline silicon film growth process usingsilicon epitaxy, particularly for thicker epitaxial films withthicknesses in excess of 30 microns is an additional issue which shouldbe addressed. Using a relatively small epitaxial film thickness (in oneembodiment, much below 30 microns) may lower the cost of epitaxy to anattractive range. However, this would present various challenges forfabrication of planar silicon thin-film solar cells. As stated, thinnerco-planar (flat) epitaxial films (e.g., in the range of much less than30 microns) produce a number of problems and challenges, including alack of film mechanical strength, constraints limiting effective surfacetexturing of thin silicon films for low surface reflectance and reducedoptical reflectance losses, relatively short optical path lengths, andreduced cell quantum efficiencies. Effective light trapping is essentialfor enhanced thin-film c-Si solar cell efficiencies. The requirement foreffective light trapping is based on a combination of front surfacetexturing and back surface mirror, while achieving sufficiently lowsurface recombination velocities (for high cell efficiencies). This isvery difficult to achieve in the co-planar (flat) c-Si thin film solarcells.

High-performance c-Si thin-film solar cells require some patterningsteps or patterned processing steps (e.g., for formation of selectiveemitter, frontside emitter or backside emitter wrap-throughmetallization contacts, backside base metallization contacts, etc.).These patterning steps are usually achieved using photolithography,screen printing, and/or shadow-mask deposition (e.g., shadow-masksputtering or evaporation) processes. The use of photolithography and/orscreen printing and/or shadow-mask deposition patterning steps usuallyincreases the manufacturing process flow complexity and cost, and mayalso detrimentally impact the fabrication yield as well as the ultimateachievable solar cell efficiency.

Therefore a need has arisen for a thin-film solar cell (TFSC) whichcorrects the problems identified above.

Yet a further need exists to address shortcomings of existing mainstreamc-Si solar cell technology. This includes reducing the amount ofpolysilicon feedstock consumed per peak watt of solar power, andeliminating the kerf losses associated with sawing and slicing; thus,substantially reducing the overall solar cell manufacturing cost.

A further need exists for innovative solar cell structures andsimplified process flows, increasing cell and module efficiencies whilesignificantly lowering the overall solar cell and module manufacturingcosts. A still further need exists for innovative c-Si solar celldesigns and simplified self-aligned manufacturing processes whichfacilitate substantial solar cell and module cost reduction even in fabswith modest production volumes, enabling low to mid-volume solar cellfabs with modest fab setup and operation costs (thus, achievingeconomies of scale for manufacturing cost reduction at substantiallylower fab volumes than the prior art fabs).

A still further need exists to address shortcomings of existing TFSCtechnology. This includes addressing difficulties associated withsufficient surface texturing of the thin planar silicon films to reducesurface reflectance losses, which currently places a limit on theminimum flat (co-planar) crystalline silicon thickness due to productionyield and cell performance considerations. A still further need existsfor effective light trapping based on a combination of front surfacetexturing and back surface mirror, while achieving low surfacerecombination velocities (for high cell efficiencies).

A still further need exists to address additional shortcomings ofexisting TFSC technologies. This includes the problem of lack ofrigidity and mechanical support of the thin film substrate during celland module processing steps, thus, necessitating the use of support orhandle substrates (made of silicon or another material) for the TFSCsubstrates. This further includes the cost of the epitaxial silicon filmgrowth process, particularly for thicker epitaxial films required forplanar crystalline silicon TFSCs. This further includes the requirementof multiple photolithography and/or screen printing and/or shadow-maskprocessing/patterning steps which usually increase the manufacturingprocess flow complexity and cost, and may also detrimentally impact thefabrication yield as well as the ultimate achievable solar cellefficiency.

SUMMARY

In accordance with the present disclosure, solar module structuresutilizing pyramidal three-dimensional thin-film solar cells (3-D TFSCs)and methods of assembling such solar module structures are provided. Thesolar module structures of the disclosed subject matter utilizingpyramidal 3-D TFSCs substantially eliminate or reduce disadvantages andproblems associated with previously developed TFSCs, both in terms ofefficiency and manufacturing cost.

According to one aspect of the disclosed subject matter, there isprovided a solar module structure comprising a top protective layer, aplurality of pyramidal 3-D TFSCs, a printed circuit boar, and aprotective back plate. Each pyramidal 3-D TFSC comprises a pyramidal 3-DTFSC substrate comprising emitter and base regions, and where thepyramidal 3-D TFSC substrate comprises a plurality of unit cells.

According to another aspect of the disclosed subject matter, there isprovided a solar module structure comprising a top glass plate, aplurality of pyramidal 3-D TFSCs, and a bottom glass plate. Eachpyramidal 3-D TFSC comprises a pyramidal 3-D TFSC substrate comprisingemitter and base regions, and where the pyramidal 3-D TFSC substratecomprises a plurality of unit cells.

According to still another aspect of the disclosed subject matter, thereis provided a solar module structure comprising a top protective layer,a plurality of pyramidal 3-D TFSCs, and a bottom protective layer. Eachpyramidal 3-D TFSC comprises a pyramidal 3-D TFSC substrate comprisingemitter and base regions, and where the pyramidal 3-D TFSC substratecomprises a plurality of unit cells.

According to still another aspect of the disclosed subject matter, thereis a provided a solar module structure. The solar module structurecomprises a top encapsulant layer, a plurality of pyramidal 3-D TFSCs, aprinted circuit board (PCB), a rear encapsulant layer and a protectiveback plate. The pyramidal 3-D TFSC comprises a pyramidal 3-D TFSCsubstrate with emitter junction regions doped base regions. Thepyramidal 3-D TFSC further includes emitter metallization regions andbase metallization regions. The pyramidal 3-D TFSC substrate comprises aplurality of pyramid-shaped unit cells.

According to another aspect of the disclosed subject matter, there is aprovided a method for assembling a solar module structure. The methodincludes a first step of patterning a frontside and a backside of adouble-sided printed circuit board coated with metallic foils accordingto desired frontside and backside interconnect layouts. The methodincludes a next step of applying a first coating layer to the rear sideof a plurality of pyramidal 3-D TFSCs. The pyramidal 3-D TFSC comprisesa pyramidal 3-D TFSC substrate with emitter junction regions doped baseregions. The pyramidal 3-D TFSC further includes emitter metallizationregions and base metallization regions. The pyramidal 3-D TFSC substratecomprises a plurality of pyramid-shaped unit cells. The method includesa next step of placing the plurality of pyramidal 3-D TFSCs on thefrontside of the double-sided printed circuit board. The method includesa next step of preparing a solar module assembly. The solar moduleassembly includes a glass layer, a top encapsulant layer, the pluralityof pyramidal 3-D TFSCs on the frontside of the double-sided printedcircuit board, a rear encapsulant layer, and a protective back plate.Finally, the method includes sealing and packaging the solar moduleassembly.

According to still another aspect of the disclosed subject matter, thereis a provided a solar module structure. The solar module structurecomprises a top glass plate, a plurality of pyramidal 3-D TFSCs, a rearpatterned cell interconnect layer, and a bottom glass plate. The 3-DTFSC comprises a pyramidal 3-D TFSC substrate with emitter junctionregions doped base regions. The pyramidal 3-D TFSC further includesemitter metallization regions and base metallization regions. Thepyramidal 3-D TFSC substrate comprises a plurality of pyramid-shapedunit cells.

According to still another aspect of the disclosed subject matter, thereis a provided a method for assembling a solar module structure. Themethod includes a first step of applying a rear patterned cellinterconnect layer to the rear side of a pyramidal 3-D TFSC. Thepyramidal 3-D TFSC comprises a pyramidal 3-D TFSC substrate with emitterjunction regions doped base regions. The pyramidal 3-D TFSC furtherincludes emitter metallization regions and base metallization regions.The pyramidal 3-D TFSC substrate comprises a plurality of prism-shapedunit cells. The method includes a next step of preparing a solar moduleassembly. The solar module assembly includes a top glass plate, theplurality of pyramidal 3-D TFSCs with the rear patterned cellinterconnect layer applied to the rear side, and a bottom glass plate.Finally, the method includes sealing and packaging the solar moduleassembly.

These and other advantages of the disclosed subject matter, as well asadditional novel features, will be apparent from the descriptionprovided herein. The intent of this summary is not to be a comprehensivedescription of the claimed subject matter, but rather to provide a shortoverview of some of the subject matter's functionality. Other systems,methods, features and advantages here provided will become apparent toone with skill in the art upon examination of the following FIGURES anddetailed description. It is intended that all such additional systems,methods, features and advantages be included within this description, bewithin the scope of the accompanying claims.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The features, nature, and advantages of the disclosed subject matter maybecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 (PRIOR ART) shows a prior art process flow for fabricatingcrystalline silicon (c-Si) thin-film solar cells (TFSCs) using planarsilicon thin-film absorber layers produced by silicon epitaxy;

FIG. 2 (PRIOR ART) shows a prior art process flow for fabrication ofsolar cells on silicon wafers including self-aligned selective emitterand metallization;

FIG. 3 (PRIOR ART) summarizes the key process steps eliminated by thecurrent disclosure, compared to the prior art;

FIG. 4 provides an overview of the 3-D TFSC substrate and solar cellfabrication process flow;

FIG. 5 shows a view of a representative example of series connections ofTFSCs of this disclosure in a solar module assembly;

FIG. 6 shows a top view of a honeycomb hexagonal-pyramid array designTFSC substrate, with a peripheral planar silicon frame;

FIG. 7 summarizes the high-level process flow and the competitiveadvantages of the current disclosure, compared to the prior art;

FIG. 8 shows a Y-Y cross-sectional view of an embodiment of a template;

FIG. 9 shows a view of the frontside layout of the printed-circuit board(PCB) used for solar module assembly;

FIG. 10 shows a top view of the backside layout of the printed-circuitboard (PCB) used for solar module assembly, showing the seriesconnection of the TFSCs;

FIG. 11 shows a backside view of the copper pattern on the PCB and isessentially similar to FIG. 61;

FIG. 12A shows an enlarged top view of the pattern on the frontside ofthe solar module printed-circuit board (PCB);

FIG. 12B shows an enlarged top view of the interconnect pattern on thebackside of the solar module printed-circuit board (PCB);

FIG. 13 shows various schematic views of a thick silicon frame, siliconframe slivers, and a representative method to produce (e.g., cut)silicon slivers;

FIG. 14 illustrates a first embodiment of a process flow for fabricationof solar modules with top protective glass plates and embedded PCBs ofthis disclosure (corresponding to the solar module structure of FIG. 15with a PCB and a TFSC mounted on the PCB);

FIG. 15 shows a cross-sectional view of a solar module (solar panel)structure (resulting from the process flow described in FIG. 14);

FIG. 16 outlines an alternative embodiment of an assembly process flowfor fabrication of solar modules (corresponding to the solar modulestructure of FIG. 17);

FIG. 17 shows a cross-sectional view of another embodiment of a solarmodule structure (resulting from the process flow described in FIG. 16);

FIG. 18 shows a view of a solar cell integrated or assembled in buildingwindows;

FIG. 19 is provided for reference for calculations;

FIG. 20 shows the ratio of the hexagonal-pyramid sidewall area to theplanar hexagonal base area (S_(hp)/S_(hb)) versus the height-to-basediagonal diameter ratio (H/d) of the hexagonal-pyramid unit cell;

FIGS. 21 and 22 shows calculated frontside aperture angles of the solarcell hexagonal-pyramid unit cell versus the height-to-base diagonaldiameter ratio (H/d) of the hexagonal-pyramid unit cell;

FIG. 23 is provided for reference for calculations;

FIGS. 24 and 25 show the ratio of the cone-shaped unit cell sidewallarea to the planar circular base area (S_(cp)/S_(cb)) versus theheight-to-base diameter ratio (H/D) of the cone-shaped unit cell andcalculated frontside aperture angle of the solar cell cone-shaped unitcell (approximation for hexagonal-pyramid units cells) versus theheight-to-base diameter ratio (H/D) of the cone-shaped unit cell;

FIG. 26 may be used for reference with an approximate analyticalcalculation of the TFSC interconnect ohmic losses, assuming a circularsubstrate with hexagonal-pyramid array of unit cells;

FIGS. 27 and 28 plot the projected (calculated) interconnect-relatedsolar cell power losses as a function of the ratio of thehexagonal-pyramid height to diagonal base dimension (H/d) for twodifferent emitter interconnect area coverage ratios on the top of the3-D solar cell substrate; while FIGS. 29 through 38 show plots forvarious values of R_(thm) and L/H.

FIGS. 39 and 40 outline embodiments of process flows for fabrication ofa template using either direct laser micromachining or photolithographypatterning;

FIGS. 41 through 44 show embodiments of process flows for fabrication ofpyramidal 3-D TFSC substrates;

FIGS. 45 and 46 show the Y-Y and Z-Z cross-sectional axes on anembodiment of a hexagonal-pyramidal (honeycomb) 3-D TFSC substrate;

FIG. 47 shows a 3-D view of a single unit cell in a hexagonal-pyramidal3-D TFSC substrate;

FIG. 48 shows a schematic ZZ cross-sectional-view of an embodiment of a3-D hexagonal-pyramid TFSC substrate;

FIG. 49 shows a schematic YY cross-sectional-view of an embodiment of a3-D hexagonal-pyramid TFSC substrate;

FIG. 50 shows a YY cross-sectional view of an embodiment of a 3-Dself-supporting hexagonal-pyramid TFSC substrate with thin square-shapedsilicon frame;

FIG. 51 shows a top view of an alternative honeycomb hexagonal-pyramidarray design TFSC substrate, with a larger thickness peripheral planarframe;

FIG. 52 shows a view of a 3-D self-supporting hexagonal-pyramid TFSCsubstrate with thick silicon frame, compared to the thin frame shown inFIG. 51;

FIG. 53 shows a top view of a 3-D TFSC substrate with a square-pyramidunit cell structure;

FIG. 54 shows a top view of a 3-D TFSC substrate with atriangular-pyramid unit cell structure;

FIG. 55 shows a top view of a 3-D TFSC substrate with an orthogonalV-groove unit cell structure;

FIG. 56 shows alternative cross sectional views of the 3-D TFSCsubstrate with an orthogonal V-groove unit cell structure, shown in FIG.55;

FIG. 57 shows a top view of a 3-D TFSC substrate with an alternativeorthogonal diagonal V-groove unit cell structure;

FIG. 58 shows alternative cross sectional views of the 3-D TFSCsubstrate with an orthogonal diagonal V-groove unit cell structure,shown in FIG. 57;

FIGS. 59 through 61 describe process flows for fabrication of a 3-D TFSCusing fire-through metallization;

FIGS. 62 through 66 describe process flows for fabrication of a 3-D TFSCusing selective plating metallization;

FIG. 67 shows a schematic view of a double-sided coater setup forself-aligned application (coating) of dopant liquid or paste layers on3-D TFSC substrate top ridges and rear surface or ridges by rollercoating and in-line curing of the applied liquid/paste layers (shown inconjunction with an integrated belt-driven process equipment);

FIG. 68 shows a view of an alternative spray coater and curing setup toperform the same processes as the roller coater and curing setup of FIG.67;

FIG. 69 shows a view of another alternative setup design usingliquid-dip coating or liquid-transfer coating to perform the sameprocesses as the roller coater and curing setup of FIG. 67 and the spraycoater and curing setup of FIG. 68;

FIG. 70 shows a cross-sectional view of a 3-D substrate (showing one ofthe hexagonal-pyramid cells) after the above-mentioned doping processstep;

FIGS. 71 shows a YY cross-sectional view after self-aligned formation ofthe emitter and base contacts and solar cell interconnects;

FIG. 72 shows a YY cross-sectional views after self-aligned formation ofthe emitter and base contacts and solar cell interconnects;

FIGS. 73 and 74 show YY cross-sectional views of the 3-Dhexagonal-pyramid solar cells (showing a single hexagonal-pyramid unitcell and several adjacent unit cells, respectively) after completion ofthe solar cell fabrication process and after soldering the rear basecontacts to the rear cell mirror (and base interconnect) plate;

FIG. 75 shows a YY cross-sectional view of the 3-D hexagonal-pyramidTFSC substrate (showing one pyramid unit cell) after self-aligned rollercoating of n-type dopant paste on the frontside honeycomb ridges, andafter curing and furnace annealing to form the selective emitter regionsand heavily-doped emitter contact regions;

FIG. 76 is similar to FIG. 75, except FIG. 76 roller coating of a p-typedopant layer and subsequent curing and anneal;

FIG. 77 shows a cross-sectional view similar to the view shown in FIG.70. However, in the embodiment shown in FIG. 77 there is only coating ofn-type dopant paste on the frontside honeycomb ridges;

FIG. 78 shows a YY cross-sectional view of a 3-D hexagonal-pyramidsubstrate after self-aligned formation of the frontside solid dopantsource layer and selective emitter, while FIG. 79 shows a ZZcross-sectional view;

FIG. 80 shows a YY cross-sectional view of a 3-D hexagonal-pyramidsubstrate after self-aligned formation of the frontside solid dopantsource layer, selective emitter, as well as the self-aligned frontsideemitter and rear base contacts, while FIG. 81 shows a ZZ cross-sectionalview;

FIG. 82 shows a view of a 3-D self-supporting hexagonal-pyramid TFSCsubstrate with thick silicon frame, compared to the thin frame shown inFIG. 72;

FIG. 83 shows a schematic quasi-3-D view of a hexagonal-pyramid unitcell after formation of the self-aligned frontside emitter contact andthe rear base contact; and

FIG. 84 shows a view of an alternative embodiment of solar cell assemblyon the rear mirror and base interconnect (the first embodiment shown inFIG. 83).

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Preferred embodiments of the present disclosure are illustrated in thedrawings, like numbers being used to refer to like and correspondingparts of the various drawings. The innovative solar cell designs andtechnologies of the current disclosure are based on the use of athree-dimensional (3-D), self-supporting, doped (in one embodiment,in-situ-doped) semiconductor thin film, deposited on and released from areusable crystalline (embodiments include monocrystalline ormulticrystalline silicon) semiconductor template.

A preferred semiconductor material for the 3-D TFSC substrate iscrystalline silicon (c-Si), although other semiconductor materials mayalso be used. One embodiment uses monocrystalline silicon as the thinfilm semiconductor material. Other embodiments use multicrystallinesilicon, polycrystalline silicon, microcrystalline silicon, amorphoussilicon, porous silicon, and/or a combination thereof. The designs hereare also applicable to other semiconductor materials such as germanium,silicon germanium, silicon carbide, a crystalline compoundsemiconductor, or a combination thereof. Additional applications includecopper indium gallium selenide (CIGS) and cadmium telluridesemiconductor thin films.

The 3-D TFSC designs and production technologies as well as associatedmodule structures and assembly approaches of this disclosure effectivelyovercome the above-mentioned problems and challenges and enablecost-reduced fabrication of very-high-efficiency solar cells and modulesusing self-aligned cell process flows without the use of anyphotolithography patterning or screen printing or shadow-mask depositionprocess steps during cell fabrication (i.e., during 3-D TFSC substrateand cell fabrication after fabrication of the reusable 3-D template).The 3-D TFSC technologies of this disclosure are based on the formationof a 3-D pyramid-array TFSC substrate structure on a low-cost reusabletemplate and its subsequent release and lift-off from the template toform a free-standing, self-supporting 3-D thin-film semiconductorsubstrate structure.

The current disclosure combines the benefits of TFSC fabrication on aproven high-efficiency crystalline silicon (c-Si) platform. The 3-D c-SiTFSC designs and technologies of this disclosure enable significantadvancements in the areas of c-Si solar cell and module efficiencyenhancement as well as manufacturing cost reduction. Based on innovativethin-film process steps, dependence on an expensive and constrainedsilicon wafer supply-chain is eliminated. Some of the unique advantagesof the cells designs and technologies of this disclosure which enableachieving ultra-high-efficiency at reduced manufacturing cost aresubstantial decoupling from the traditional solar PV silicon supplychain, performance enhancement, cost reduction, and reliabilityimprovement.

The disclosed subject matter improves solar cell efficiency by using a3-D c-Si film as an absorber layer in conjunction with highly efficientlight trapping. Use of the crystalline silicon absorber layer leveragesknown solar cell manufacturing techniques and supply chain, whilereducing absorber layer thickness (e.g., reduced by a factor of ten ormore compared to silicon wafers used for wafer-based solar cells). Thedisclosed method and system eliminates or substantially reducesphoto-degradation and enhances open-circuit voltage (V_(oc)) of cells.In addition, the disclosed method and system provides efficientfrontside and rear side light-trapping in conjunction with a highlyreflective rear mirror for maximum absorption of incident solar flux.Also, the disclosed method and system provides a selective emitter toenhance blue response and external quantum efficiency, with minimalshadowing of the cell and reduced ohmic losses due to a unique foldedemitter metallization contact design and improved module assembly.

Manufacturing cost is reduced by decreasing silicon usage (by asignificant factor, e.g., 3× to over 10×), with thinner deposited c-Sifilms also reducing the finished solar module energy payback time toless than 1 to 2 years. Manufacturing cost is further reduced byeliminating wire sawing and related kerf losses associated withmainstream solar cell wafer manufacturing technology. Manufacturing costis still further reduced by using self-aligned processing without anylithography or patterning steps used during the substrate and cellfabrication process flow, and a reduced number of fabrication processsteps, with improved yield and cycle time. Production cost is stillfurther reduced by using a simplified interconnection and cell-moduleassembly process and lightweight monolithic modules.

Operational reliability is improved by using thinner silicon films,eliminating photo-degradation and reducing temperature coefficients.Operational reliability is further improved by using a simpledistributed high-conductance electrical interconnection, minimizingfield failures. Operational reliability is still further improved byeliminating module glass cover (for glassless module assembly), thusreducing cost and facilitating field installation and operation.Operational reliability is still further improved by reducing the numberof manufacturing process steps and process variations using in-linemanufacturing process control.

The current disclosure reduces the solar module cost per watt for theuser (by at least 30% to 50%) and cuts balance-of-system (BOS) andinstallation costs for the integrators and installers. This may offermajor benefits to the global grid-tied end-users and solar systeminstallers and integrators. The current disclosure reduces the moduleintegration and installation cost and installed solar cell system costper W_(p) for the user, thereby lowering finished system cost per W_(p).The current disclosure increases module efficiency, with higher moduleefficiency resulting in lower BOS cost. The lower installed solar cellsystem cost results in reduction of the economic break-even time to alower fraction of the system lifetime, from roughly ½ to ⅓ for currentbest-of-breed c-Si solar cell systems to less than ¼ to ⅛ for theembodiments of this disclosure. The current disclosure reduces energypay-back time (EPBT) from 3 to 7 years for best-of-breed c-Si solar cellsystems to less than 1 to 2 years for the embodiments of thisdisclosure. Reduced EPBT substantially increases the net lifetime energyoutput (in kWh) for field-installed modules. The cell designs and moduleassemblies of this disclosure also provide stable degradation-free fieldoperation over an extended time (e.g., 30 to 40 year life of themodule), further increasing the net lifetime electrical energy output.Module manufacturing costs are expected to be 30% to 65% lower than thatof the leading high-performance c-Si solar cells/modules at the time ofmarket entry. This may shorten the ROI break-even time for the userscompared to the current industry roadmap and projections. Furtherbenefits include increased field performance stability and reliabilityand reduced environmental impact (non-toxic materials and shortenedEPBT). Further, the cell and module designs of this disclosure are idealfor grid-tied applications where it is advantageous to maximizeelectricity generation from a limited building rooftop or façade area.

The absorber silicon film thickness of the current disclosure may be avalue in the range of roughly 1 to 30 microns, where a thinner siliconlayer is preferred for less material consumption (in one embodiment, inthe range of 1 to 10 microns). Even after taking into account theeffective surface area increase due to the 3-D geometric structure ofthe 3-D TFSC substrates, the 3-D TFSC substrates of this disclosureconsume substantially less silicon material than the state-of-the-artwafer-based c-Si solar cells. Moreover, there are no sawing or kerflosses. Similarly, there is no requirement for saw damage removal sincethe 3-D crystalline silicon film is process-ready upon release from thereusable template. This substantially reduces the solar cell costassociated with silicon consumption. The self-supporting 3-D epitaxialsilicon thin film is deposited on and released from a low-cost reusablecrystalline (monocrystalline or multicrystalline) silicon substrate(template). The template may be reused numerous times before beingreconditioned or recycled. The template may even be chosen from the muchlower cost metallurgical-grade c-Si since any metallic impurities areprevented from contaminating the 3-D crystalline silicon film.

FIG. 3 summarizes the overall crystalline solar cell fabrication processflow of prior art techniques and highlights the specific stepseliminated by the current disclosure, compared to the prior art. FIG. 4summarizes the overall cell and module fabrication process flow and thecompetitive advantages of the current disclosure, compared to the priorart. As highlighted here, the current disclosure enables fabrication of3-D TFSC substrates and3-D TFSCs, thus, substantially reducingconsumption of semiconductor absorber material (e.g., silicon) and thecell and module manufacturing costs.

FIG. 5 shows a view of a representative example of series connections ofTFSCs of this disclosure in a solar module assembly 100. This exampleshows 24 squared-shaped cells 102 connected in series (in a 6×4 array).The electrical connections in series are shown by arrows between theadjacent cells connected in series. Module power input 104 and output106 leads are also shown. In actual module assemblies, the numbers ofcells may be smaller or larger and the cells may be connected in seriesor in a combination of series and parallel. As mentioned earlier, seriesconnection of the cells within the module assembly allows for steppingup the DC voltage for the DC-to-AC inverter (and also limiting the DCcurrent of the solar modules for ease of module installation in thefield and reliability of the module-to-module electrical connections).The printed-circuit-board (PCB) based module assembly of this disclosuresupports any number of cells assembled in a module and any electricalconnection configuration (series, series/parallel combination, orparallel). The TFSCs and modules of this disclosure may providerelatively lightweight solar modules with areas from less than 1 m² toseveral m² (e.g., 10 m²) for various applications. The cells connectedin series within a module assembly are chosen based on sorting to bematched in terms of their photogenerated current (e.g., short-circuitcurrent I_(sc), and/or maximum-power current I_(m)).

FIG. 6 shows a top view 110 of a honeycomb hexagonal-pyramid arraydesign TFSC substrate 112, with a peripheral planar silicon frame 114.The design includes a periodic array of high-aspect-ratio (orlow-aperture-angle) hexagonal-pyramid unit cells 116. In one embodiment,frame length (S) 118 is 125 mm to over 200 mm. Silicon Frame 114 mayhave the same thickness as the TFSC substrate 112 or may be much thicker(e.g., silicon frame thickness=5 to 500 microns). The top surface of theframe is also preferably used as the top solar cell interconnect (it ismetallized along with the top honeycomb contacts, and is electricallyconnected to the honeycomb contacts). In one embodiment, the width 120of the frame 114 is roughly 125 microns to 1 mm. The film thickness 122of the TFSC substrate is roughly 2 to 30 microns, preferably 2 to 10microns. Typically, there are millions (or as few as thousands) of thesehexagonal-pyramid unit cells 116 form a large-area (e.g., 200 mm×200 mm)TFSC substrate 112.

FIG. 7 provides an overview of the 3-D TFSC substrate and cellfabrication process flow. Focusing on the top of FIG. 7 illustrating the3-D TFSC substrate fabrication, note that the first step in this processflow uses a pre-fabricated template. The template with a pre-fabricated3-D trench or groove pattern may be used for formation of 3-D TFSCsubstrates, which are then used in the formation of 3-D TFSCs,substantially eliminating or reducing disadvantages and problemsassociated with previously developed TFSCs and the wafer-basedcrystalline silicon cell technologies. The template is capable of beingused numerous times (e.g., tens to hundreds of times) to fabricatenumerous 3-D TFSC substrates before being reconditioned or recycled. Inone embodiment, the template may be used hundreds of times to fabricate3-D TFSC substrates before being recycled. The template may be reusedfor as long as it remains relatively free of dislocations and/or for aslong as it maintains an acceptable trench or groove pattern with widthsand surface conditions within acceptable control limits (e.g., as gaugedby in-line metrology).

FIG. 8 shows a Y-Y cross-sectional view of a template 130 showinghexagonal-pyramid trenches 132 between posts 134, formed in thesubstrate fronstide 136 using the process flows described above. Thebottom 138 of the trenches 132 connects to through-wafer backside,preferably small-diameter, holes 140 which connect to the templatebackside 142. In one embodiment, the holes 140 are 1 to 10 microns indiameter. The holes 140 are used for subsequent wet etching of thesacrificial layer and 3-D silicon film release and lift-off. Thetemplate 130 has dimensions of L 144 (in one embodiment, 0 to 25 micronsalthough it may also be much larger up to several hundred microns), 3-Dunit cell height H 146, pyramidal angle a 148, and unit cell aperturediameter h 150.

In the next section, various embodiments of this disclosure for makingsolar modules suitable for building rooftops and façades, centralizedpower generation, and other applications are described. Usually solarmodules are made by arranging a plurality of solar cells and connectingthem in series (series electrical connections) within a solar moduleassembly protected by a top glass layer and a rear protective materiallayer such as Tedlar. The cells may be connected in series in order tostep up the DC voltage (while maintaining the solar module current atthe same level at the level of the cell current) to facilitatehigh-efficiency DC-to-AC power conversion.

The solar module structures and assembly methods of this disclosure arebased on the use of a printed-circuit board (PCB) to assemble the 3-DTFSCs in a closely packed array and to connect the cells (in oneembodiment in series) using the PCB plate within a module assembly. ThePCB plate may have a single patterned metal (in one embodiment, copper)interconnect layer on the top of the PCB or two patterned copper layerson the top and rear surfaces of the PCB plate. FIG. 9 shows a view 160of the frontside silver-coated copper layout of the printed-circuitboard (PCB) used for solar module assembly (the square islands serveboth as rear mirrors (if no integrated mirror is used withsingle-aperture cells, or if the cells are dual-aperture cells withoutbase layers) and base interconnects; the peripheral square-shaped copperbands connect to the wrap-around emitter contact at the TFSC peripheralframe rear side; copper-filled via plugs connecting select regions ofthe PCB frontside and backside are shown as small circles). This exampleis shown for an array of 24 TFSCs arranged in 4 rows of 6 cells in eachrow (the PCB may be designed for any number and various arrangements ofTFSCs). The PCB conductor (copper or aluminum) thickness may be in therange of roughly 10 to over 100 microns to provide high electrical andthermal conductivities. The PCB also serves as an effective heat sink tominimize temperature cycling of the TFSCs in operation. The PCB materialmay be selected to be a lightweight, high-strength material (such ascarbon composite materials used in aerospace industry), or even arelatively thin flexible material. The larger-area square-shapedsilver-coated copper regions 162 are connected to the TFSC rear baseregions (bottoms of the rear base layers for the single-aperture cellsor the bottom ridges of the dual-aperture cells for the dual-aperturecells). The peripheral silver-coated copper lines 164 are electricallyconnected to the TFSC emitter contact metallization regions.

FIG. 10 shows a top view 170 of the backside (optionally silver-coated)copper layout of the printed-circuit board (PCB) used for solar moduleassembly, showing the series connection of the TFSCs. The PCB backsidemay also include thin-film shunt diodes for shade protection of theTFSCs (as shown in FIG. 9). The copper-filled via plugs (shown ascircles) connect the PCB frontside and backside metallization patternsin the corresponding areas. While the example shown here is forconnecting 24 TFSCs in series on a solar panel, similar PCB designmethodology may be applied to configure and connect any number of cellsin any desired arrangements on the module. The frontside view of thisPCB is shown in FIG. 9. This example is shown for an array of 24 TFSCsarranged in 4 rows of 6 cells in each row (the PCB may be designed forany number and various arrangements of TFSCs), all connected in series.The PCB conductor (copper or aluminum) thickness may be in the range ofroughly 10 to over 100 microns to provide high electrical and thermalconductivities. The PCB also serves as an effective heat sink tominimize temperature cycling of the TFSCs in operation. The PCB materialmay be selected to be a lightweight, high-strength material (such assuitable carbon composite materials used in aerospace industry). FIG. 10also shows power input Lead 104 (first cell's p-lead) and power outputlead 106 (last cell's n-lead).

FIG. 11 shows a backside view 180 of the copper pattern on the PCB andis essentially similar to FIG. 10. This picture also shows the use ofprotective thin-film shunt diodes mounted on the PCB backside pattern(for cell shadow protection).

FIG. 12A shows an enlarged top view 190 of the silver-coated copperpattern (the pad for mounting one cell) on the frontside of the solarmodule printed-circuit board (PCB) used for rear mirror and also emitterand base interconnects for one of the TFSCs (relative dimensions are notshown to scale). FIG. 12A shows dimensions of L₁ 192 and L₂ 194 (in oneembodiment, 150 millimeters to greater than 200 millimeters, whereL₂=L₁₊₂(W+S)). S 196 may be on the order of 25 to 250 microns. The widthof the peripheral copper conductor band (W) 198 may be on the order of50 to 500 microns. The copper-filled via plugs 200 are shown as circles(connecting the interconnect patterns on the PCB frontside and backsidein a pre-designed arrangement in order to connect the TFSCs in series orin any other desired arrangement such as series/parallel; therepresentative example shown here is for connecting all the cells inseries in order to step up the module open-circuit voltage). The viaplug 200 diameters may be on the order of roughly 50 to 500 microns (andmay be smaller than W 198). The large central square pad serves both asthe rear cell mirror and also base interconnect plane (connecting to thehexagonal-pyramid base contact metallization). The number of vias in thecenter square (p-region contact) (N) 202 may be on the order of hundredsto thousands. The number of vias in the peripheral line (n-regioncontact) (M) 204 may be on the order of tens to hundreds (or eventhousands). The vias on the peripheral line contacting the TFSC emitter(n) regions are placed on three sides. The PCB conductor (copper oraluminum) thickness may be in the range of roughly 10 to over 100microns to provide high electrical and thermal conductivities. The PCBplate also serves as an effective heat sink to minimize temperaturecycling of the TFSCs in field operation. This FIGURE shows one of thecopper interconnect/mirror pads shown in the full module PCB array ofFIG. 9.

FIG. 12B shows an enlarged top view 210 of the silver-coated copperinterconnect pattern on the backside of the solar module printed-circuitboard (PCB) used for emitter and base electrical interconnects for acouple of adjacent TFSCs of this disclosure (a portion of the PCB view).FIG. 12B shows the PCB backside silver-coated copper interconnectpattern for TFSCs 1 and 2 in the array. The copper pattern here is shownfor connecting the TFSCs in series to step up the module open-circuitvoltage. FIG. 12B shows dimensions of L₁′ 212; peripheral emitter(n-region) connector linewidth W′ 214 (in one embodiment, 2 to 10millimeters); spacing between the center base (p-region) connector plateand the peripheral emitter (n-region) connector line S′ 216 (in oneembodiment, 100 microns to 1 millimeter). Note that L₁′ 212 is less thanL₁ from FIG. 12A by roughly 2 to 10 millimeters. This enables largerperipheral emitter (n-region) connector linewidth and substantiallyreduced ohmic losses on the PCB backside.

The 3-D TFSC substrates of this disclosure may utilize peripheral thicksilicon frames, both for added mechanical support and also to facilitateformation of wrap-through or wrap-around emitter contact metallization(for ease of solar module assembly). The thick silicon frame may beseparately made from very low-cost silicon material (such asmetallurgical grade or reclaim silicon wafers). FIG. 13 shows variousschematic views 220 of the thick silicon frame, the silicon frameslivers, and representative method to produce (e.g., cut) siliconslivers from very-low-cost round (e.g., reject silicon frommicroelectronics) or square-shaped (or rectangular) cast silicon (orreclaim Si) substrates. The slivers may be made of very low-costcrystalline or multicrystalline silicon such as metallurgical-grade castSi. A round 222 or square-shaped 224 silicon wafer (e.g., a 200 mm×200mm cast metallurgical-grade silicon substrate) may be used to producehundreds of silicon slivers 226 by a cutting process such as lasercutting (four slivers used to make a thick silicon frame for a 3-D TFSCsubstrate by a welding process such as electron-beam welding).

These slivers 226 may be used to make the thick silicon frames for thesubstrates shown above. The separately fabricated thick silicon framemay then be integrally attached to the 3-D TFSC substrates, inembodiment before 3-D thin-film cell processing, by one of the followingtechniques: electron-beam welding at several peripheral spots/junctions;attachment during the 3-D TFSC substrate fabrication silicon depositionby placing the peripheral thick silicon frame on the template andallowing seamless attachment of the thick silicon frame to the 3-D TFSCsubstrate by the silicon deposition process; or a clean cured epoxy.

Top view 228 shows a thick silicon frame to be fused to the 3-D TFSCsubstrate. The silicon frame thickness 230 is roughly 50 to 500 microns.There are welded (e.g., e-beam-welded) joints 232 (four welded joints),where L 234 is roughly 150 to 300 millimeters, and where W 236 isroughly 100 to 1000 microns. The slivers 226 may also have through-holes(shown in view 1068) to help with the wrap-through/wrap-around emittermetallization contacts.

The PCB assembly described above may be used to create the final solarmodule assembly in a number of ways (with or without a frame, with orwithout top tempered glass, etc.).

FIG. 14 illustrates a first embodiment of a process flow 240 forfabrication of solar modules with top protective glass plates andembedded PCBs of this disclosure (corresponding to the solar modulestructure of FIG. 15 with a PCB and a TFSC mounted on the PCB). Thismanufacturing flow is compatible with a fully automated module assemblyline. This module assembly flow is based on the use of a double-sidedprinted-circuit board (PCB) with the cell rear mirrors/baseinterconnects on the PCB topside (silver-coated patterned copper on thePCB topside). For hexagonal-pyramid 3-D TFSCs with rear base layers andintegrated/embedded (or attached) rear mirrors fabricated prior tomodule assembly (e.g., hexagonal-pyramid cells with rear base layers andthin-film rear mirrors deposited on the rear surfaces of the rear baselayers using PVD or plating or roller coating/spray coating and curing),the patterned PCB copper layer does not have to be coated with ahigh-reflectivity mirror material (silver). In step 242, module assemblystarts with a double-sided PCB coated with copper foils on bothfrontside and backside. The PCB area should support the desirednumber/layout of TFSCs (e.g., ≧1 m², with a copper foil thickness oneach side of roughly 10 to over 100 microns). Step 244 involves PCBinterconnect patterning and silver flash coating (the latter if neededfor PCB rear mirror). The PCB frontside and backside copper foils arepatterned according to the desired frontside and backside interconnectlayouts. Copper patterns are flash coated with a thin layer of highlyreflective silver (and/or aluminum). A highly reflective diffuse mirrormay be used, though a specular mirror may also be used. Step 246involves cell preparation for automated TFSC placement and soldering.The rear metallized side of the TFSCs is roller coated (or spray coatedor dip coated) with lead-free solder or an electrically conductive andthermally-conductive epoxy paste. Step 248 involves automated TFSCplacement and soldering (or curing of epoxy). TFSCs are automaticallypicked and placed in a closely-packed array on the frontside of the PCB.The rear side of each cell sits on its designated site on the frontsideof the double-sided PCB with patterned copper interconnects. The TFSCrear hexagonal-pyramid base interconnect is soldered to the PCBfrontside silver-coated patterned copper islands using thermal orultrasonic soldering. In case of using epoxy instead of solder, theepoxy layer is cured using thermal and/or IR/UV curing. The protectivethin-film shunt diodes are mounted and soldered (or epoxied) on the PCBbackside. An optional step is to flash coat the metal regions with athin layer of highly reflective silver. Step 250 involves final solarmodule assembly and lamination. A stack of low-reflection tempered (inone embodiment, also textured) top glass, an encapsulant layer, thecell-mounted PCB, another encapsulant layer and a Tedlar or polyvinylfluoride back sheet is prepared. Next, the module stack assembly ishermetically sealed and packaged, for instance, using vacuum-pressurelamination.

FIG. 15 shows a cross-sectional view 260 of a solar module (solar panel)structure (resulting from the process flow described in FIG. 14) with aprotective back plate 262 made of a proven prior art material (e.g.,Tedlar or polyvinyl fluoride film); a rear encapsulant layer 264 (EVA),a 2-sided printed-circuit board (PCB) 266 of this disclosure with rearpatterned electrical interconnects 268 and top patterned electricalinterconnects 270; cell rear mirrors and TFSCs 272 with rear base andwrap-around (or wrap-through) emitter contacts mounted on the frontsideof the PCB, a top encapsulate layer (EVA) 274, and ananti-reflection-coated (ARC) tempered glass (in one embodiment, texturedtempered glass) 276 (from rear to top), with greater than 98%transmission, with sputtered or sprayed or liquid-coated anti-reflectioncoating). This module structure may be assembled as a hermeticallysealed package either as a frameless module or with a frame (e.g., madeof aluminum). In one embodiment, the module assembly is a framelessassembly (also for reduced materials energy content and reduced energypayback time).

FIG. 16 outlines an alternative embodiment of an assembly process flow280 for fabrication of reduced cost and reduced weight (lightweight)solar modules (corresponding to the solar module structure of FIG. 17).This flow is compatible with a fully automated module assembly. Thisprocess flow shows the assembly process without the use of a thick glassplate (thus, further reducing the weight, cost, and energy payback timeof the solar modules of this disclosure) and without an EVA encapsulantlayer on the top of the cells. The module topside (the frontside ofassembled cells) is covered with a hard protective glass-type layer (ifdesired, also including a top ARC layer) with a combined thickness onthe order of tens to hundreds of microns. As deposited, this frontsideprotective layer is effectively textured as a result of the 3-Dstructure of the TFSCs. The top layer may be formed by a liquid coatingtechnique (e.g., spray coating, liquid-dip coating, or roller coating)following by a thermal or UV curing process. The thermal (or UV) curefor the liquid-spray-coated (or liquid-dip coated or roller coated)protective/AR layers may be performed as a single step together with thevacuum-pressure thermal lamination process. This embodiment results in alightweight module assembly with reduced materials consumption, reducedcost, and reduced energy payback time. Step 282 (providing PCB)corresponds to step 242 in FIG. 14; step 284 (PCB patterning and silverflash coat) corresponds to step 244; step 286 (cell preparation)corresponds to step 246; and step 288 (automated TFSC placement)corresponds to step 248. Step 290 involves solar module lamination. Astack of the cell-mounted PCB, an encapsulant layer, and a back sheet isprepared. Next, a suitable hermetic sealing/packaging process such asvacuum-pressure lamination is performed. Step 292 involves deposition ofthe solar module frontside protective coating (which may beautomatically textured as deposited and provides efficient lighttrapping for effective coupling to the TFSCs) layer and an optional ARClayer. The frontside of the solar panel is coated with a thin layer ofprotective material (e.g., a glass-type transparent material) and anoptional top anti-reflection coating (ARC) layer using a suitablecoating method. This coating (roughly tens to hundreds of microns) maybe performed using liquid spray coating, liquid roller coating,liquid-dip coating, plasma spray coating or another suitable method.Next, a thermal/UV curing process is performed.

FIG. 17 shows a cross-sectional view 300 of another embodiment of asolar module structure (resulting from the process flow described inFIG. 16). Instead of a top encapsulate layer (EVA) 274, and ananti-reflection-coated (ARC) tempered glass 276, as shown in FIG. 15,there is a single frontside protective layer and anti-reflective coatinglayer 302. The frontside protective layer and anti-reflective coating(ARC) layer 302 is formed by liquid spray coating/curing, liquid rollercoating/curing, liquid-dip coating/curing, plasma spray coating, oranother suitable low-temperature coating technique. This frontsideprotective coating and ARC layer 302 is effectively textured for thecoating layer as deposited as a result of the 3-D structure of the TFSCs(thus, no separate texturing process is needed). This is due to the factthat the coating layer may have dips (low points) over the TFSChexagonal-pyramid cavities and peaks (high points) over thehexagonal-pyramid emitter ridges. The frontside protective layer andanti-reflective coating layer 302 may have a combined thickness in therange of tens to hundreds of microns. In one embodiment, the thicknessmay be approximately 30 to 300 microns. In addition to providing ananti-reflection coating (ARC) function, the stacked frontsideprotective/ARC layer provides excellent protection againstweather/elements and force impact (e.g., hail impact) in actual outdoorfield operation. Since the frontside coating is effectively andautomatically textured as a result of the 3-D structure of the TFSCs,the use of a separate ARC layer on the frontside coating is optional.The textured coating may provide effective light trapping in thefrontside coating for effective coupling of a very high fraction (e.g.,greater than 95%) of the incident solar light intensity to the TFSCs.The frontside protective layers may also provide an optical waveguidingfunction to eliminate or reduce any reflection losses associated withthe top emitter contact metallization.

FIG. 18 shows a view 310 of a solar cell integrated or assembled inbuilding windows. The solar cell can allow partial visible lighttransmission (e.g., with transmission on the order of 10% to 30%) bycreating an array of holes or slot openings in the 3-D hexagonal-pyramidsolar cell substrate. In one embodiment, the cell has a regular array ofholes or slot openings to allow for 5% to 20% light transmission. ThisFIGURE has a magnified view of a portion of the solar glass with thehexagonal-pyramid cells (thus, the relative dimensions of thehexagonal-pyramid cell and the solar glass are not shown to scale). FIG.18 shows frontside TFSC hexagonal emitter interconnects 312 andself-aligned backside hexagonal base contact 314. The distance 316between the top glass plate 318 and bottom glass plate 320 may bebetween 1 and 12 millimeters. The hexagonal-pyramid cell parameters maybe designed to allow for a desired level of light transmission throughthe cell (e.g., roughly 10% to 90%). The level of average lighttransmissivity can be controlled by the aspect ratio of the TFSCs.

The following section outlines various calculations related to thedisclosed subject matter.

For given thin silicon film thickness and substrate size (e.g., 200mm×200 mm substrate size) values, the actual amount (e.g., amount asmeasured by the total silicon surface area, volume, or weight) ofsilicon material used in the 3-D hexagonal-pyramid substrate structureis actually larger than that of a co-planar (flat) substrate with thesame dimensions (e.g., 200 mm×200 mm).

FIG. 19 is provided for reference for the following calculations. B isthe mid-point between H₂ and H₃; A is the mid-point between H₅ and H₆;H₁H₄=H₃H₆=H₂H₅=d; and AB is the hexagonal unit cell aperture horizontaldistance (h):

h=(√3/2)d

Frontside aperture angle α is the angle defined by A-T-B, and frontsideaperture angle β is the angle defined by H₆-T-H₃, and can be calculatedas follows:

α=2 tan⁻¹ [(√3.d)/(4H)]

β=2 tan⁻¹ [d/(2H)]

The surface area of the cone pyramid base (S_(hb)):

S _(hb)=[(3√3)/8].d ²

The surface area of the cone pyramid sidewall (S_(hp)):

S _(hp)=[(3√3)/8 ].d ².√[1+(16/3).(H/D)²]

Therefore, the effective surface area enlargement factor (S_(hp)/S_(hb))is:

S _(hp) /S _(hb)=√[1+(16/3).(H/d)²]

To achieve very efficient light trapping within the 3-D TFSC structureand very low effective surface reflectance with a reasonable (i.e., notexcessive) area enlargement factor of S_(hp)/S_(hb), the aperture angles(α and β) are chosen to be preferably in the range of around 20° toaround 40°.

FIG. 20 shows the ratio of the hexagonal-pyramid sidewall area to theplanar hexagonal base area (S_(hp)/S_(hb)) versus the height-to-basediagonal diameter ratio (H/d) of the hexagonal-pyramid unit cell. Thepreferred H/d range for near-optimal aperture angles is shown betweendashed lines (H/d roughly 1.5 to 3.0). This results in sidewall-to-basearea ratio on the order of roughly 4 to 7.

FIGS. 21 and 22 shows calculated frontside aperture angles (α and β) ofthe solar cell hexagonal-pyramid unit cell versus the height-to-basediagonal diameter ratio (H/d) of the hexagonal-pyramid unit cell.

FIG. 23 is provided for reference for the following calculations. Ahexagonal-pyramid unit cell of the 3-D substrate may be approximated bya cone (with the same height as a hexagonal-pyramid and the same basearea as a hexagonal-pyramid). The aperture angle (φ) is:

φ=2 tan⁻¹ [D/(2H)]

The surface area of cone pyramid base (S_(cb)) is:

S _(cb)=(πD ²)/4

The surface area of cone pyramid sidewall (S_(cp)) is:

S _(cp)=[(πD ²)/4].√[1+(2H/D)²]

Therefore, the ratio of the sidewall surface area S_(cp) to the top basesurface area S_(cb) is:

S _(cp) /S _(cb)=√[1+(2H/D)²]

FIGS. 24 and 25 show the ratio of the cone-shaped unit cell sidewallarea to the planar circular base area (S_(cp)/S_(cb)) versus theheight-to-base diameter ratio (H/D) of the cone-shaped unit cell andcalculated frontside aperture angle φ of the solar cell cone-shaped unitcell (approximation for hexagonal-pyramid units cells) versus theheight-to-base diameter ratio (H/D) of the cone-shaped unit cell. TheseFIGURES provide plots of aperture angle and surface area ratio for acone-shaped pyramid, whereas FIGS. 20 to 23 show these plots for ahexagonal-pyramid unit cell. The results (plots) for a cone-shapedpyramid unit cell (approximation of a hexagonal-pyramid unit cell) arefairly comparable to those for the hexagonal-pyramid unit cell.

One important consideration in the TFSC and module interconnects is thetotal power loss associated with the electrical interconnects in theTFSCs and the solar module assembly. The hexagonal-pyramid 3-D c-Si TFSCand PCB-based module designs of this disclosure effectively address thisissue, resulting in very low interconnect ohmic losses in the cells andwithin the module. This feature (in conjunction with the highlyefficient packing of the TFSCs on the PCB-based solar module assembly)substantially narrows the efficiency gap between the TFSCs and the solarmodule assembly in the technology of this disclosure.

The next section relates to the basic calculations of the emittercontact metallization ohmic losses in the hexagonal-pyramid 3-D TFSCs ofthis disclosure. The calculations of ohmic losses for emitter contactmetallization are also applicable to the base contact metallization.However, since several embodiments of this disclosure mount the 3-DTFSCs on patterned printed circuit boards (PCBs), the base contactmetallization is electrically connected in a planar format to a veryhigh conductivity copper pad; this substantially reduces the baseinterconnect ohmic losses (compared to the emitter interconnect ohmiclosses). Therefore, in practical embodiments of this disclosure, theinterconnect ohmic losses are dominated by the emitter contactmetallization.

FIG. 26 may be used for reference with an approximate analyticalcalculation of the TFSC interconnect ohmic losses, assuming a circularsubstrate with hexagonal-pyramid array of unit cells base on the celldesign embodiments of this disclosure. Since the overall cellinterconnect ohmic losses are dominated by the top emitter contactmetallization, the ohmic power loss due to the hexagonal emitter contactmetallization is calculated as a function of cell current at maximumpower and emitter contact metal vertical height coverage ratio L/d(ratio of the height of emitter contact metal coverage on the pyramidsidewall to the pyramid unit cell long hexagonal diagonal dimension).The analytical calculations shown here were used to produce the plotsshown in the following FIGURES (FIGS. 27-38). The calculations performedand trends obtained for round substrates are also approximatelyapplicable to square-shaped TFSC substrates.

For the following calculations: I₀is the total cell current at peakpower; A=(πA²)/4, total cell area (shown for round cell);J₀=(4.I₀)/(πa²), cell current density; R_(thm) is the sheet resistanceof top honeycomb contact metal; C is the effective flat surface coverageof honeycomb contact with vertical height L; and R_(eff)=R_(thm)/C,where R_(eff) is the effective flat surface sheet resistance of topmetal contact.

Based on this, the interconnect ohmic losses as maximum cell power are:

P_(I) ≅ (R_(thm)I₀²)/{8π(S_(hp)/S_(hb))[1 − (1 − L/H)²]}$P_{I} \cong {\left( {R_{thm}I_{0}^{2}} \right)/\left\{ {8{{\pi \left\lbrack \sqrt{1 + {\left( {16/3} \right)\left( {H/d} \right)^{2}}} \right\rbrack}\left\lbrack {1 - \left( {1 - {L/H}} \right)^{2}} \right\rbrack}} \right\}}$

FIGS. 27 and 28 plot the projected (calculated) interconnect-relatedsolar cell power losses in the 3-D TFSCs of the disclosed subject matteras a function of the ratio of the hexagonal-pyramid height to diagonalbase dimension (H/d) for two different emitter interconnect areacoverage ratios on the top of the 3-D solar cell substrate. For H/d=2.0,the ohmic power loss experienced as a result of extracting the maximumpower from a 400 cm² solar cell is projected to be around 0.1 to 0.2 W(depending on the emitter metal coverage shown for the two plots). Sincethe maximum solar cell power in this example is assumed to be around 8W, the ohmic power losses for the solar cell interconnects (dominated bythe emitter current collection) is projected to be between 1.25% and2.5%. The PCB metal pattern can be designed such that the PCBinterconnect ohmic power losses are much smaller than theabove-mentioned solar cell interconnect power losses. Thus, the totalohmic power losses can be kept to well below 2%. This means that usingthe solar cell and module technology of the disclosed subject matter,the efficiency gap between the solar cells and the solar modules can bereduced to well below 2%. Thus, with a solar cell efficiency of 23%, wecan have a high degree of confidence that we will achieve a solar moduleefficiency of at least 21%. Both FIGURES show the calculated solar cellohmic power loss for large-area cells with 400 cm² area and 12A currentat maximum power (roughly 8 W_(p) max power assumed). R_(thm) is thesheet resistance of the emitter contact/metal layer (e.g., Ag layer or astack of Ag on a refractory metal layer) on the honeycomb hexagonalridges (0.0075 Ω square for both graphs). The ratio L/H (here, 0.05 inFIGS. 27 and 0.025 in FIG. 28) is the ratio of the vertical coverageheight of the emitter metallized contact on the honeycomb ridges to theheight of the hexagonal-pyramid unit cell.

The following FIGURES show plots for various values of R_(thm) and L/H.FIG. 29 shows R_(thm) of 0.002 Ω square and L/H of 0.05; and FIGS. 30shows R_(thm) of 0.002 Ω square and L/H of 0.01. FIG. 31 shows R_(thm)of 0.005 Ω square and L/H of 0.01; and FIGS. 32 shows R_(thm) of 0.005 Ωsquare and L/H of 0.02. FIG. 33 shows R_(thm) of 0.005 Ω square and L/Hof 0.05; and FIG. 34 shows R_(thm) of 0.005 Ω square and L/H of 0.1.FIG. 35 shows R_(thm) of 0.01 Ω square and L/H of 0.05; and FIGS. 36shows R_(thm) of 0.01 Ω square and L/H of 0.02. FIG. 37 shows R_(thm) of0.003 Ω square and L/H of 0.02; and FIGS. 38 shows R_(thm) of 0.003 Ωsquare and L/H of 0.05.

As noted in FIG. 7, the first step in manufacturing a solar moduleassembly is the fabrication of a template.

In the following section, alternative embodiments of process flows forfabricating templates using either lithography and etch techniques orlaser micromachining (or laser drilling) are described. The templatesare then used and reused numerous times to fabricate the 3-D TFSCsubstrates for 3-D TFSC fabrication, which are then used in theformation of 3-D TFSCs, which are used in thin-film solar moduleassemblies of the disclosed subject matter.

Templates may be fabricated using electronic-grade silicon wafers,solar-grade silicon wafers, or lower-cost metallurgical-grade siliconwafers. Moreover, templates made of silicon can be fabricated eitherusing monocrystalline or multicrystalline silicon wafers. The startingtemplate wafer may either be a standard polished wafer (after saw damageremoval) or even a lower grade wafer immediately after wire sawing(without saw damage removal). The latter may further reduce the cost ofthe templates. The relatively low cost of each template is spread overnumerous 3-D TFSC substrates, resulting in much lower TFSC substrate andfinished module costs compared to the standard state-of-the-art (e.g.,200 microns thick) solar-grade monocrystalline and multicrystallinesilicon wafers and associated modules.

For further explaining how a template is fabricated, FIG. 39 shows anembodiment of a process flow 340. The process begins with step 342,where an unpatterned monocrystalline silicon or multicrystallinesilicon, either square-shaped or round substrate (e.g., 200 mm×200 mmsquare or 200-mm round) is provided. The starting template wafer may bea wafer prepared by wire saw either with or without saw damage removal(the latter may further reduce the cost of template). The startingtemplate wafer may also be made of a lower purity (and lower cost)metallurgical-grade silicon. In one embodiment, the substrate is roughly200 to 800 microns thick. Optionally, step 342 includes performinggettering on a low-cost metallurgical-grade silicon and/or performing asurface texturing etch (e.g., using isotropic acid texturing by amixture of nitric acid and hydrofluoric acid, or using alkalinetexturing in KOH/IPA) to create an optional textured template surface.Step 344 involves the use of programmable precision laser micromachiningto form the desired periodic array of deep trenches. This process may beperformed in a controlled atmospheric ambient based on either physicalablation or a combination of physical ablation and laser-assistedchemical etching. Step 346 involves template surface preparation andcleaning. This process includes stripping the patterned photoresistlayer from the substrate. The template substrate is then cleaned in awet bench prior to subsequent thermal deposition processing to form theTFSC substrates. Such cleaning may involve DRIE-induced polymer removal(using a suitable wet etchant such as a mixture of sulfuric acid andhydrogen peroxide) followed by an isotropic silicon wet etch (such as ina mixture of nitric acid and hydrofluoric acid) in order toisotropically remove a thin layer (e.g., on the order of 10 to 500nanometers) of silicon from the trench sidewalls and bottoms. This mayremove any surface and buried contaminants, such as any surface andembedded metallic and/or polymeric/organic contaminants introduced bythe deep RIE (DRIE) process, from the sidewalls and bottoms of theDRIE-produced template trenches. Template processing may complete aftera deionized (DI) water rinse and drying. Optionally and if desired, thetemplate wafer may also go through a standard pre-diffusion (orpre-thermal processing) wafer cleaning process such as a so-called RCAwet clean prior to the above-mentioned DI water rinsing and drying.Another optional surface preparation step (either performed instead ofor after the wet isotropic silicon etch process) includes performing ashort thermal oxidation (e.g., to grow 5 to 100 nanometers ofsacrificial silicon dioxide), followed by wet hydrofluoric acid (HF)oxide strip (to remove any residual contaminants from the patternedtemplate). If no optional oxide growth/HF strip is used, an optionaldilute HF etch may performed to remove the native oxide layer and topassivate the surface with hydrogen (forming Si—H bonds) in preparationfor subsequent 3-D TFSC substrate fabrication. After the completion ofstep 346, the resulting template may then be used and reused multipletimes to fabricate 3-D (e.g. hexagonal-pyramid) TFSC substrates.

An alternative embodiment of a process flow 350 for patterning of atemplate is outlined in FIG. 40, which uses photolithography and etchinstead of direct laser micromachining. Step 352 (providing anunpatterned substrate) corresponds to step 342 of FIG. 39. Step 354 usesphotolithography patterning (in one embodiment, using a relativelylow-cost contact or proximity aligner/patterning) to produce a maskpattern such as hexagonal-pyramid pattern in photoresist (i.e.,interconnected hexagonal openings in the photoresist layer). The processsequence includes the formation of an oxide and/or nitride (optional)layer, photoresist coating (e.g., spin-on or spray coating) andpre-bake, photolithography exposure through a hexagonal-array mask, andphotoresist development and post-bake. One embodiment includes anoptional hard mask layer (SiO₂ and/or SiN_(x); for example, a thinthermally grown oxide layer can be used as an optional hard mask) belowthe photoresist (although the process may be performed without the useof any hard mask layer by placing the photoresist coating directly onsilicon). When using a hard mask layer, the exposed portions of the hardmask layer are etched after photoresist patterning (thus, forminghexagonal openings). Such etching of the exposed hard mask layer may besimply performed using a wet etchant such as hydrofluoric acid for oxidehard mask or using plasma etching. Step 356 involves formation ofhexagonal-pyramids using anisotropic plasma etch; where a high-rate deepreactive ion etch (DRIE) process forms a closely-packed array of deep(e.g., 100 to 400 microns) hexagonal-pyramid shaped trenches (i.e.,pyramidal trenches with sloped sidewalls) in silicon. The photoresistand/or oxide and/or nitride hard mask layer(s) are used for patterntransfer from the patterned photoresist layer to silicon substrate. Inone embodiment, the deep RIE (DRIE) process parameters are set toproduce a controlled-angle hexagonal-pyramid sidewall slope. The RIE isallowed to produce small-diameter (e.g., less than 5 microns) holes atthe bottom of pyramids by punching through the substrate backside.Alternatively, separate small-diameter backside holes may be formedwhich connect to the bottom tips of the pyramidal trenches. Step 358(surface preparation and cleaning) corresponds to step 346 of FIG. 39.After the completion of step 358, the resulting template may then beused and reused multiple times to fabricate 3-D (e.g. hexagonal-pyramid)TFSC substrates.

The templates described above may be used to fabricate 3-D TFSCsubstrates for use in 3-D TFSCs. All of the embodiments shown in FIGS.41 through 44 use sacrificial layer formation (e.g., porous siliconsacrificial layer) and trench-fill deposition processes (e.g., epitaxialsilicon deposition) which may be highly conformal, for conformalformation of the sacrificial (porous silicon) layer and subsequentseamless void-free filling of the trenches with a semiconductor absorberlayer such as in-situ-doped (e.g., in-situ boron doped) monocrystallineor multicrystalline silicon layer. One embodiment uses a patternedsingle-crystal (monocrystalline) silicon or multicrystalline silicon(mc-Si) square-shaped (or round) template, with dimensions ofapproximately 150 mm×150 mm to over 200 mm×200 mm. Alternativeembodiments may use much lower cost metallurgical-grade or solar-gradesilicon.

FIG. 41 shows an embodiment of a process flow 360 for fabrication ofself-supporting, free-standing 3-D hexagonal-pyramid TFSC substrates.The process uses a lift-off 3-D thin-film release process based on ahighly selective etch process to remove an interfacial sacrificial layer(of Ge_(1-x)Si_(x)) without any appreciable etching of silicon. TheGe_(1-x)Si_(x) layer may be a single layer with a constant Ge fractionor a multilayer (e.g., 2 to 3 layers) structure with varying Gefractions. In step 362, a patterned square-shaped template is provided.This template has already been processed to form an array ofhexagonal-pyramid trenches on its frontside 136 along with an array ofthrough-wafer holes 140 from trench bottoms 138 to backside 142. Step364 involves a multi-layer blanket epi in an epitaxial reactor. Step 364first involves an H₂ or GeH₄/H₂ in-situ cleaning, which is performedafter a standard pre-epi wet clean. Next, a thin sacrificial epi layeris deposited on the frontside only. In one embodiment, Ge_(x)Si_(1-x),is used for the sacrificial epi layer and is between 10 and 200nanometers. Next, a doped silicon epi layer is deposited on thefrontside only. In one embodiment, the layer is p-type, boron-doped andhas a thickness between 2 and 20 microns. Step 366 involves 3-D TFSCsubstrate release. A highly selective isotropic wet or dry etch ofGe_(x)Si_(1-x), is performed, with very high selectivity with respect tosilicon. In one embodiment, a mixture of hydrofluoric acid, nitric acidand acetic acid (HNA) is used to etch the Ge_(x)Si_(1-x) layer.Alternatively, a mixture of ammonia, peroxide, and water(NH₄OH+H₂O₂+H₂O) may be used. This process releases the silicon epilayer as a hexagonal-pyramid 3-D TFSC substrate, which may then be usedfor subsequent 3-D TFSC fabrication.

Depending on the emitter doping type (n-type or p-type), the in-situbase doping type is chosen to be p-type (e.g., boron) or n-type (e.g.,phosphorus). The embodiments shown provide examples of boron-dopedhexagonal-pyramid 3-D TFSC substrates which may be used to fabricateTFSCs with n-type, phosphorus-doped selective emitters. In analternative embodiment, all the doping polarities may be inverted,resulting in phosphorus-doped hexagonal-pyramid 3-D TFSC substrateswhich may be used for fabricating cells with boron-doped selectiveemitters.

FIG. 42 shows an alternative embodiment of a process flow 370 forfabrication of self-supporting, free-standing 3-D hexagonal-pyramid TFSCsubstrates. The same process flow may be used for fabrication of 3-DTFSC substrates with other unit cell structures (square pyramid,triangular pyramid, orthogonal V-grooves, orthogonal diagonal V-grooves,etc.). This process uses a suitable sacrificial material layer (e.g.,silicon dioxide) to facilitate the 3-D TFSC substrate release andlift-off (or removal from the template). Laser crystallization may beused to crystallize the amorphous silicon or polysilicon layer. Step 372(providing a template) corresponds to step 362 above. Step 374 involvessacrificial layer deposition, where a thin sacrificial layer (e.g.,SiO₂) is deposited preferably by APCVD (alternatively, use LPCVD orPECVD or even thermal oxidation). In one embodiment, this layer isbetween 20 nm and 200 nm of SiO₂. In step 376, doped (e.g., p-type)silicon layer (preferably doped amorphous silicon or polysilicon) isdeposited on oxide-coated substrate (top only) by a CVD process such asPECVD (e.g., 2 to 20 microns thick; boron-doped). Step 378 involvesdepositing a thin sacrificial layer (e.g., SiO₂) as a top protectivelayer, preferably by chemical-vapor deposition process such as APCVD(alternatively, use LPCVD or PECVD or even thermal oxidation). In oneembodiment, this layer may be 5 nm to 50 nm of SiO₂. In step 380, lasercrystallization (preferably starting from a silicon frame at substrateedge) of doped silicon layer is performed, using a preferablysquare-shaped single-crystal silicon frame as laser crystallization seed(edge-to-center crystallization). Step 382 involves releasing the 3-DTFSC substrate, by performing a highly selective isotropic HF wetetching of the sacrificial SiO₂ layer (with high selectivity withrespect to silicon) to etch off the oxide release layer between thep-type 3-D silicon film and the template (this process lifts the 3-Dhexagonal-pyramid silicon film). For all embodiments, releasing the 3-Dfilm may be aided by applying a small mechanical stress (e.g., waferwarpage) or applying ultrasonic or megasonic agitation during therelease etch.

FIG. 43 shows an alternative embodiment of a process flow 390 forfabrication of self-supporting, free-standing 3-D hexagonal-pyramid TFSCsubstrates. The same process flow may be used for fabrication of 3-DTFSC substrates with other unit cell structures (square pyramid,triangular pyramid, orthogonal V-grooves, orthogonal diagonal V-grooves,other types of pyramids, etc.). This process uses a sacrificial ordisposable material layer (e.g., silicon dioxide) to facilitate the 3-DTFSC substrate release and. Laser crystallization may be used tocrystallize the amorphous silicon or polysilicon layer. Step 392(providing a template) corresponds to step 362 above. Step 394(sacrificial layer deposition) corresponds to step 374 above. Step 396involves performing a backside wet etching of SiO₂ to re-open theoxide-filled substrate bottom holes. Step 398 (doped silicon deposition)corresponds to step 376 above; and step 400 (sacrificial layerdeposition) corresponds to step 378. In step 402, laser crystallizationof the doped silicon layer is performed, using the single-crystalislands at the bottom holes of hexagonal-pyramids as lasercrystallization seeds. Step 404 (3-D TFSC substrate release) correspondsto step 382 above.

FIG. 44 shows an alternative embodiment of a process flow 410 forfabrication of self-supporting, free-standing 3-D hexagonal-pyramid TFSCsubstrates. The same process flow may be used for fabrication of 3-DTFSC substrates with other unit cell structures (square pyramid,triangular pyramid, orthogonal V-grooves, orthogonal diagonal V-grooves,other pyramidal unit cell structures, etc.). This process useselectrochemical silicon etching to form low and/or high porosity poroussilicon layer(s) prior to semiconductor (e.g., silicon) epitaxy. Step412 (providing a template) corresponds to step 362 above. Step 414involves Electrochemical Silicon Etching in a Wet Bench to produce asingle-layer porous silicon or a bilayer stack of a top layer oflow-porosity porous silicon on a layer of high-porosity porous silicon,using electrochemical HF etching (also known as electrochemicalanodization of silicon to form porous silicon). The porous silicon layeror layer stack may be formed either by direct electrochemical etching ofthe template substrate or by first depositing a thin layer of epitaxialsilicon and then converting the thin silicon epi layer to sacrificialporous silicon using electrochemical etching. In step 416, siliconepitaxy is performed in an epitaxial reactor to form preferablymonocrystalline silicon on top of the porous silicon sacrificial layeror layer stack, with the following in-situ process steps performed: H₂in-situ clean; deposition of doped (e.g., p-type) silicon epi (top only)(e.g., 2 to 20 microns thick; boron-doped). Finally, in step 418, the3-D TFSC substrate is released by either applying a mechanical stress tothe substrate (e.g., by applying a slight warpage to the substrate), orsimply by selective wet etching of the sacrificial porous silicon layer(or layer stack) using a suitable etchant (such as HF+H₂O₂ or TMAH oranother suitable selective porous silicon etchant).

The following FIGURES show embodiments of 3-D TFSC substrate structures.The crystalline silicon film thickness is in the range of 2 to 30microns, and preferably in the lower-end range of 2 to 10 microns. Thisis substantially less (by a factor of roughly 20× to 100×) than thestate-of-the-art silicon solar cell wafer thickness (roughly 200microns).

FIG. 45 shows a top view 420 of a regular (equilateral)hexagonal-pyramid 3-D TFSC substrate 112, formed using the process stepsoutlined above. Each hexagonal unit cell 116 contains hexagonal unitcell points (H₁, H₂, H₃, H₄, H₅, and H₆) 422, 424, 426, 428, 430, and432, with the bottom tip (which will form the back contact) of thehexagonal-pyramid shown as point 434. FIG. 45 further shows thehexagonal-pyramid 3-D TFSC substrate sidewalls 436; the diagonaldimension of the unit cell hexagon (d) 438; and hexagonal unit cellhorizontal distance (h) 440. In one embodiment, the hexagonal-pyramid3-D TFSC substrate sidewalls 436 are between 2 and 30 microns thick.

FIG. 46 shows a bottom view 450 of the TFSC substrate 112 shown in FIG.45. In this view, the hexagonal-pyramid rear (bottom) tips 434 are shownat the centers of the hexagons. The honeycomb hexagons are the bottomviews of the top honeycomb ridges of the 3-D TFSC substrate.

FIG. 47 shows a quasi 3-D view 460 of an individual hexagonal-pyramidunit cell 116 of the TFSC substrate 112 shown above. The top hexagonalopening of the unit cell 116 forms the frontside self-alignedinterconnected contacts of the thin-film solar cell (TFSC). Also shownin this view is the width (W) 122 of the semiconductor film forming thesidewalls of the hexagonal-pyramid cell 116 and the height (H) 462 ofthe cell 116, and d/2 464.

When referring to the following FIGURES, the YY and ZZ axes are shown inFIGS. 45 and 46. FIG. 48 shows a ZZ cross-sectional-view 470 of the 3-Dhexagonal-pyramid TFSC substrate 112, showing the hexagonal-pyramid topaperture angles β 472. The bottom tips 434 of the triangles are the reartips of the hexagonal-pyramids (where the base contacts will be placed).The solid line shows the 3-D TFSC substrate thin-film silicon layer,with thickness 122 (in one embodiment, roughly 1 to 25 microns thick).FIG. 49 shows a YY cross-sectional-view 480 of the hexagonal-pyramidTFSC substrate 112, showing the hexagonal-pyramid top aperture angles α148.

FIG. 50 shows a YY cross-sectional view 490 (not to scale) of anembodiment of a 3-D self-supporting hexagonal-pyramid TFSC substrate 112with thin square-shaped silicon frame 114 (like the frame 114 shown inFIG. 6), with thickness the same as the film thickness 122. The siliconframe width (W) 120 is between 50 and 250 microns. Note that the widthof the top hexagonal honeycomb silicon ridges 492 is preferably muchsmaller than h 440 and H 462. In one embodiment, the width of the tophoneycomb ridges 492 is roughly 0.5 microns to less than 5 microns.

FIG. 51 shows a top view 500 of an alternative honeycombhexagonal-pyramid array design TFSC substrate 112, with a largerthickness 120 peripheral planar silicon frame 114.

FIG. 52 shows a YY cross-sectional view 510 (not to scale) of anembodiment of a 3-D self-supporting hexagonal-pyramid TFSC substrate 112with thicker square-shaped silicon frame 114 (like the frame 114 shownin FIG. 51). Note that the thickness of the frame is not the same as thefilm thickness 122. Instead, the thick silicon frame may be roughly 100to 500 microns thick. The silicon frame width (W) 120 is between 50 and250 microns. The thick peripheral silicon frame may be preferably madeof low-cost metallurgical-grade silicon and may be attached to the 3-DTFSC substrate using one of the following methods: (i) thick siliconframe placed on the reusable silicon template and fused to the 3-Dthin-film silicon film during the epitaxial silicon growth process; (ii)e-beam welding of the 3-D TFSC substrate to the thick silicon frame(after lift-off/release of the 3-D TFSC substrate from the reusablesilicon template); or (iii) thermal bonding (under clamping pressure) ofthe thick silicon frame to the 3-D TFSC substrate. The thick siliconframe maybe used for enhanced mechanical support and rigidity of the 3-DTFSC substrate.

The previous section illustrated a preferred embodiment of ahexagonal-pyramid TFSC substrate. Alternative embodiments may usealternative designs for the unit cells such as inverted pyramid unitcells with polygon bases, including square pyramid, triangular pyramid,etc.; other embodiments may include 3-D TFSC substrates with V-groove ororthogonal V-groove patterns, etc. For example, FIG. 53 shows a top view520 of a 3-D TFSC substrate with a square-pyramid unit cell structure.The primary difference between this pattern and the preferred embodimenthexagonal-pyramid unit cell pattern is the top base (or pyramidaperture) geometry (square base versus hexagonal base for the invertedpyramid unit cells). The vertical height and base area values of thesquare pyramid unit cells are comparable to those of thehexagonal-pyramid unit cells (similar considerations apply). Anotherexample is shown in FIG. 54, which shows a top view 530 of a 3-D TFSCsubstrate with a triangular-pyramid unit cell structure. The primarydifference between this pattern and the preferred embodimenthexagonal-pyramid unit cell pattern is the top base or pyramid aperturegeometry (triangular base versus hexagonal base for the inverted pyramidunit cells). The vertical height and base area values of the triangularpyramid unit cells are comparable to those of the hexagonal-pyramid unitcells (similar considerations apply).

FIG. 55 shows a top view 540 of a 3-D TFSC substrate with an orthogonalV-groove unit cell structure. The orthogonal V-groove unit cellpreferably has four adjacent rectangular or square-shaped arrays ofV-grooves, with the V-grooves in each pair of adjacent sub-unit cellsrunning perpendicular to each other. A 3-D TFSC substrate uses a largenumber of the orthogonal unit cells shown above. The ranges for heightand width of the V-grooves in the orthogonal V-groove unit cells aresimilar to the height and diameter ranges of the hexagonal-pyramid unitcells, respectively. Each square-shaped sub-unit cell (four sub-unitcells shown above produce one orthogonal V-groove unit cell) may havetens to hundreds of V-grooves. The orthogonal V-groove structureprovides additional mechanical rigidity compared to the standardV-groove structure with parallel V-grooves on the entire substrate. Thedotted lines depict the V-groove trench bottoms whereas the solid linesshow the top ridges of the V-grooves. FIG. 56 shows XX 550, YY 560, andZZ 570 cross sectional views of the 3-D TFSC substrate with anorthogonal V-groove unit cell structure, shown in FIG. 55.

FIG. 57 shows a top view 580 of a 3-D TFSC substrate with an alternativeorthogonal diagonal V-groove unit cell structure, compared to theorthogonal V-groove unit cell structure shown in FIG. 55. The orthogonaldiagonal V-groove unit cell preferably has four adjacent rectangular orsquare-shaped arrays of V-grooves, with the V-grooves in each pair ofadjacent sub-unit cells running perpendicular to each other. Theorthogonal V-groove structure provides additional mechanical rigiditycompared to the standard V-groove structure with parallel V-grooves onthe entire substrate. FIG. 58 shows YY 590, XX 600, and ZZ 610 crosssectional views of the 3-D TFSC substrate with an alternative orthogonaldiagonal V-groove unit cell structure, shown in FIG. 57.

As noted in FIG. 7, the templates described above may be used forformation of 3-D thin-film TFSC substrates, which are then used in theformation of 3-D TFSCs, which are used in thin-film solar moduleassemblies of the disclosed subject matter.

The following section details the process of using the 3-D TFSCsubstrates described above to fabricate 3-D thin-film solar cells(TFSCs). Specifically, the following FIGURES illustrate embodiments ofprocess flows using alternative methods of fire-through metallizationand selective plating metallization. These process flows do not use anyphotolithography or screen printing processes. The 3-D hexagonal-pyramidstructural design of the solar cell substrate enables self-alignedprocessing throughout the entire cell process flow. The emitter and basecontacts and metallized regions cover relatively small fractions of thefrontside emitter and backside base areas, respectively. As indicated,selective emitter and base doping is achieved by self-alignedapplication of the n-type and p-type dopant pastes to the top and bottomof the 3-D hexagonal-pyramid substrate, preferably using double-sidedroller coating method. The 3-D substrate is then cured and moves on to abelt furnace to form the n⁺ selective emitter on the frontside and thep³⁰ ⁻doped base on the rear side of the 3-D substrate. A preferredn-type dopant source is phosphorus and a preferred p-type dopant sourceis boron.

FIGS. 59 through 61 describe process flows using fire-throughmetallization, while FIGS. 62 through 66 describe process flows usingselective plating metallization.

Process flow 620 shown in FIG. 59 describes an embodiment usingfire-through metallization, with a forming gas anneal (FGA) step aftercopper (Cu) or silver (Ag) plating. Step 622 starts with a p-type 3-Dsilicon TFSC substrate. Step 624 involves selectively coating the topportions of the hexagonal-pyramids (in one embodiment, the top 2 to 10microns in height) with n-type liquid or paste dopant source. Coating isperformed by self-aligned roller coating using dopant paste/liquid, orliquid-dip coating by dipping in a controlled liquid dopant sourcedepth. The dopant layer is then dried/cured (250° C. to 400° C. or UV).Step 626 involves formation of self-aligned emitter contacts. The bottomportions of the pyramid tips are selectively coated (in one embodiment,about 2 to 10 microns in height) with p-type liquid or paste dopantsource. Coating is performed by self-aligned roller coating using dopantpaste/liquid, or liquid-dip coating by dipping in a controlled liquiddopant source depth. The dopant layer is then dried/cured (250° C. to400° C. or UV). Step 628 involves self-aligned selective emitterformation, where the top n⁺p and n⁻⁺p junctions and rear p⁺ tips areconcurrently formed by an anneal (e.g., 800° C. to 950° C.). The dopantlayer drying and annealing may be performed sequentially in a singlebelt furnace. Preferably, the substrate may be annealed face down on aheated planar surface, or with pairs of substrates in frontsideface-to-face contact, in order to facilitate gas-phase doping formationof n⁺ regions. In step 630 (Surface passivation and ARC), the dopantsource layer and native oxide are stripped (in one embodiment, usingdilute HF). Thin oxide is grown by steam oxidation (e.g., 3 to 10 nm @850° C. to 950° C.); and SiN_(x) ARC is then deposited by PECVD. Bothlayers are formed on both silicon sides, with PECVD-SiN_(x) alsoproviding H passivation of silicon. In step 632 (Self-alignedmetallization), the top portions of the hexagonal-pyramids areselectively coated (to a height less than the dopant source) with metal(Ag) paste using self-aligned roller coating, then dried and cured. Thebottom tips of the hexagonal-pyramids are then selectively coated on therear side with metal (Al or Ag) paste by self-aligned roller coating,then dried and cured. In step 634 (Self-aligned metallization (firing)),the front (Ag) and rear (Al, Ag) metallized regions are formed by firingthrough the thermal oxide/PECVD SiN_(x) layers. Step 636 involves anoptional self-aligned metallization step, where an FGA is performed(e.g., 300° C. to 450° C.) to reduce front and rear interconnectresistance values and help with surface/bulk passivation. Step 638involves an optional self-aligned metallization step, where Cu or Ag areselectively/concurrently deposited (e.g., 1 to 5 microns) on themetallized top honeycomb ridges and bottom hexagonal-pyramid tips byplating. Metallized regions are then flash coated with Ag. In step 640,the solar cell backside metallized hexagonal-pyramid tips are solderedto a Cu or Ag mirror plate or foil (may be perforated), then flashcoated with Ag. The rear mirror also serves as the rear electricalconnector. Finally, in step 642, the solar cells can be packaged intosolar modules/panels.

An alternative fire-through metallization process flow 650 is describedin FIG. 60. Emitter contacts and interconnects are made on the tophoneycomb ridges whereas the base contacts are made on rearhexagonal-pyramid tips. In this embodiment the rear base contact regionsare heavily doped by Al during the fire-through process (no separate p⁺rear base doping used by boron dopant source). Forming gas anneal (FGA)performed after Cu and/or Ag plating. Step 652 (providing the substrate)corresponds to step 622 in FIG. 59; and step 654 (selectively coatingthe top portions) corresponds to step 624. However, step 626 (selectivecoating of bottom portions) is not performed next. Instead, subsequentsteps 656 to 670 correspond to steps 628 to 642.

Another alternative fire-through metallization process flow 680 isdescribed in FIG. 61. Compared to process flow 650 in FIG. 60, theforming gas anneal (FGA) step is performed before Cu and/or Ag plating.Specifically, steps 664 and 666 from FIG. 60 are reversed, noted insteps 694 and 696 of FIG. 61.

As noted above, FIGS. 62 through 66 describe process flows usingselective plating metallization.

Process flow 710 shown in FIG. 62 describes an embodiment usingselective plating metallization. Step 712 (providing a substrate)corresponds to step 622 in FIG. 59; step 714 (selectively coating thetop portions) corresponds to step 624; step 716 (selectively coating thebottom portions) corresponds to step 626; step 718 corresponds to step628; and step 720 corresponds to step 630. However, step 722(self-aligned metallization) involves selectively etching the topportions of the hexagonal-pyramids (to a height less than the dopantsource) by self-aligned roller coating in etchant paste, followed byrinsing, and repeating the process on the rear hexagonal-pyramid tips.Step 724 (self-aligned metallization) involves concurrently forming thefront and rear metallized regions by a single plating process (e.g., Ag,Ni, Pt, Ti, Co, Ta). Step 726 involves an optional self-alignedmetallization step where a forming gas anneal (FGA) is performed (e.g.,300 to 450° C.), to reduce front and rear interconnect resistance valuesand help with surface/bulk passivation. Step 728 involves self-alignedmetallization where Cu or Ag is selectively/concurrently deposited(e.g., 1 to 5 microns) on the metallized top honeycomb ridges and bottomhexagonal-pyramid tips by plating. Step 730 (solder) and step 732(proceed with packaging) are the same as above.

An alternative selective plating metallization process flow 740 isdescribed in FIG. 63. No p⁺ dopant paste coating is used for rear basecontact doping. The rear base p⁺ contact doping is performed by Aldoping in the base contact regions using an anneal process after rearbase tip Al metallization using plating. Step 742 (providing thesubstrate) corresponds to step 712 in FIG. 62; and step 744 (selectivelycoating the top portions) corresponds to step 714. However, step 716(selective coating of bottom portions) is not performed next. Instead,step 746 (self-aligned selective emitter) is performed next,corresponding to step 718. Step 748 (Surface passivation and ARC)corresponds to step 720; and step 750 (self-aligned metallization)corresponds to step 722. In step 752, the rear metallized regions areselectively formed by an Al rear plating process (e.g., dip plate therear/backside only). In step 754, metal (Ag, Ni) is selectively platedon front exposed honeycomb n⁺⁺ doped regions and rear Al-metallizedregions. Step 756 involves performing an FGA (300 to 450° C.) to formAl-doped rear p⁺ tips. The FGA reduces front and rear interconnectresistance values and helps with surface/bulk passivation. Step 758(plating) corresponds to step 728; step 760 (solder) corresponds to step730; and step 762 (proceed with packaging) corresponds to step 732.

Another alternative selective plating metallization process flow 770 isdescribed in FIG. 64. In contrast to the embodiment is FIG. 62, noroller paste etching process is used here. Instead the process here usesselective etching of dopant source layers. Steps 772 (providing asubstrate), step 774 (selectively coating the top portions), and step776 (selectively coating the bottom portions) correspond to steps 712 to716 in FIG. 62. Step 778 (self-aligned selective emitter) involvesconcurrently forming the top n⁺p and n⁺⁺p junctions and rear p⁺ tips byan anneal (e.g., 800 to 950C.), sequentially first in an inert (Ar, N₂)and then oxidizing (steam) ambient to grow 5 to 50 nm of thermal oxide.The dopant layer drying and anneal may also be performed sequentially ina single belt furnace. Step 780 involves selectively etching the dopantsource layers while removing only a small portion of oxide, using a wetetch with high etch selectivity compared to thermal oxide. Step 782(plating) corresponds to step 724; and step 784 (plating) corresponds tostep 728. Step 786 involves soldering (Pb-free solder) backsidemetallized hexagonal-pyramid tips to an Al mirror plate or foil (may beperforated), then flash coating with Ag. The rear mirror also serves asthe rear electrical connector. Step 788 involves concurrently depositingARC layer (e.g., low-temperature PECVD SiN_(x)) on both front and rearsurfaces. The ARC layer may also include spectral down-conversion. TheARC layer also helps with additional H passivation. Note that theperipheries of frontside solar cell and rear mirror should be maskedduring PECVD to facilitate cell/module interconnects. Step 790 (FGA)corresponds to step 724, and step 792 (packaging) is the same as above.

Another alternative selective plating metallization process flow 800 isdescribed in FIG. 65. Compared to process flow 550 from FIG. 64, thisprocess uses only one high temperature (anneal and oxidation) processstep. Steps 802 to 816 are the same as steps 772 to 786 in FIG. 64.However, in step 818, an ARC layer (e.g., low temperature PECVD SiN_(x))is deposited on solar cell front surface only. Steps 820 and 822 are thesame as steps 790 and 792 in FIG. 64.

Another alternative selective plating metallization process flow 830 isdescribed in FIG. 66. Like process flow 800 from FIG. 65, this processuses only one high temperature (anneal and oxidation) process step.Steps 832 to 840 correspond to steps 802 to 810 from FIG. 65. However,step 812 (plating) is not performed. Instead, only plating step 814 isperformed, corresponding to step 842. Steps 844 to 850 correspond tosteps 816 to 822 from FIG. 65.

The above process steps may be performed on integrated in-line processequipment. For example, FIG. 67 shows a view 860 of a setup forperforming the two process steps of liquid/paste coating and UV or IRcuring prior to furnace anneal, allowing for subsequent formation ofselective emitter and base regions after anneal in an in-line diffusionfurnace. This integrated in-line process equipment allows forself-aligned formation of dopant liquid or paste coating on the 3-D TFSCsubstrate top ridges and rear ridges by roller coating. Roller coatingmay be performed using an atmospheric-pressure, belt-driven coating andcuring equipment integrated in line with a diffusion furnace. In oneembodiment, the top ridges are coated with n-type dopant liquid/paste;the rear ridges are coated with p-type dopant liquid/paste.

The 3-D TFSC substrate 862 is shown moving in 864 on input conveyor belt866. The rotating top rollers 868, with top roller pads 870, apply acontrolled downward force to coat the top ridges with n-type paste. Therotating rear rollers 872, with rear roller pads 874, apply a controlledupward force to coat the rear ridges with p-type paste. Multilayermaterials may be coated on each side of the 3-D TFSC substrate byapplying (or flowing) a different liquid or paste material to eachroller on the top 868 and/or rear 872 set of rollers. The 3-D TFSCsubstrate 862 next moves into the curing area where the dopantliquid/paste layers are concurrently formed using a curing lamp 876which uses IR or UV curing beams 878. The 3-D TFSC substrate 862 is nextshown moving out 880 to the output conveyor belt 882, which may move thesubstrate 862 to an in-line diffusion furnace, where the n⁺ and p⁻contacts and selective emitter regions are concurrently formed.

A similar roller coater setup may be properly configured and used forapplying metal liquid/paste coatings (e.g., silver and/or aluminumliquid or paste sources), curing the metal liquid/paste source, andperforming subsequent thermal anneal in an in-line atmospheric furnace(resistively-heated or lam-heated furnace) for fire-throughmetallization in order to form the emitter and base contactmetallization (and whenever applicable, also to form the aluminum-dopedp⁺⁺ base contact regions).

FIG. 68 shows a view 890 of an alternative setup design to perform thesame processes as the roller coater/curing/furnace setup of FIG. 67. Thesetup in FIG. 68 may be used for self-aligned formation of dopant sourceliquid/paste coating on the 3-D TFSC substrate top ridges and rearridges by angled spray coating. This setup also may utilize an in-lineatmospheric-pressure coating and curing and diffusion equipmentconfiguration which can be easily integrated with an in-line diffusionfurnace. As with the roller coater setup in FIG. 67, multilayermaterials may be coated on each side of the substrate by using multiplesets of spray nozzles connected to different liquid sources (not shownhere) and applying (or flowing) a different liquid source material toeach nozzle on the top and/or rear set of spray nozzles. This is analternative technique to the roller coating system shown in FIG. 67. Inone embodiment, the top ridges are coated with n-type dopantliquid/paste (such as phosphorus); the rear ridges are coated withp-type dopant liquid/paste (such as boron). Referring to FIG. 68, the3-D TFSC substrate 862 is shown moving in 864 on input conveyor belt866. Angled nozzles 892 spray n-type dopant liquid onto the surface at asharp angle with respect to the surface (nozzles cover wafer width).This n-type dopant liquid comes from an n-type liquid dopant source andnozzle reservoir/pump 894. Angled nozzles 896 spray p-type dopant liquidonto the surface at a sharp angle with respect to the surface (nozzlescover wafer width). This p-type dopant liquid comes from a p-type liquiddopant source and nozzle pump 898. The 3-D TFSC substrate 862 next movesinto the curing area where the dopant liquid/paste layers areconcurrently formed using a curing lamp 876 which uses IR or UV curingbeams 878. The 3-D TFSC substrate 862 is next shown moving out 880 tothe output conveyor belt 882, which may move the substrate 862 to anin-line diffusion furnace, where the n⁺ and p⁺ contacts and selectiveemitter regions are concurrently formed.

The angled spray technique limits the vertical height of theliquid/paste coating to a portion of the ridges and prevents the liquidsource from coating the inner parts of the hexagonal pyramid cavitysidewalls and/or rears. This type of in-line (or another drive method)processing system may also be used for applying metal source liquid(e.g., silver and/or aluminum source liquid) for fire-throughmetallization applications as well as applying liquid etchant forselective etching of dielectrics (e.g., oxide and/or solid dopant sourcelayer) from the top and/or rear hexagonal-pyramid ridges.

FIG. 69 shows a view 900 of another alternative setup to perform thesame processes as the in-line roller coater/curing setup of FIG. 67 andthe in-line spray coater/curing setup of FIG. 68. The setup in FIG. 69may be used for self-aligned formation of dopant liquid/paste coating onthe 3-D TFSC substrate top ridges and rear ridges by liquid-dip coating.This setup also may utilize an in-line atmospheric-pressure coating andcuring equipment configuration to be attached to the input stage of anin-line diffusion (or fire-through) furnace.

In one embodiment, the top ridges are coated with n-type dopantliquid/paste (such as phosphorus); the rear ridges are coated withp-type dopant liquid/paste (such as boron). The 3-D TFSC substrate 862is shown moving in 864 on input conveyor belt 866. Liquid film dispensercontaining n-type liquid dopant source 902 applies a controlledthickness n-type liquid dopant film 904. This n-type dopant liquid comesfrom n-type liquid dopant source and liquid level and depth controller906. Liquid film dispenser containing p-Type liquid dopant source (withperipheral air levitation) 908 applies a controlled thickness p-typeliquid dopant film 910. This p-type dopant liquid comes from p-typeliquid dopant source and liquid level and depth controller 912. The 3-DTFSC substrate 862 next moves into the curing area where the dopantliquid/paste layers are concurrently formed using a curing lamp 876which uses IR or UV curing beams 878. The 3-D TFSC substrate 862 is nextshown moving out 880 to the output conveyor belt 882, which may move thesubstrate 862 to an in-line diffusion furnace, where the n⁺ and p⁺contacts and selective emitter regions are concurrently formed.

As in the setups in FIGS. 67 and 68, multilayer materials may be coatedon each side of the 3-D TFSC substrate by using multiple sets ofliquid-dip applicators (not shown here) and applying (or flowing) adifferent liquid source material to each liquid-dip applicator on thetop and/or rear set of applicators. This type of processing system mayalso be used for applying metal liquid for fire-through metallization aswell as applying liquid etchant for selective etching of dielectrics(e.g., oxide and/or solid dopant source layer) from the top and/or rearridges.

The following section shows cross-sectional views of the TFSC substrateduring various stages of the process flows outlined above. In thefollowing FIGURES, the relative dimensions are not shown to scale.

FIG. 70 shows a cross-sectional view 920 of the 3-D substrate 112(showing one of the hexagonal-pyramid cells 116) after theabove-mentioned doping process step in a suitable process equipment suchas an in line belt-driven equipment (roller coating or spray coating orliquid-dip coating or another liquid/paste-transfer coating of dopantliquids/pastes, drying/curing, and diffusion furnace anneal). In regardto the n-type (e.g., phosphorus) dopant paste or liquid 922 covering thetop portion of the hexagonal ridges 492, the single furnace annealprocess in the diffusion furnace (e.g., at roughly 800° C. to 950° C.)produces more heavily-doped contact diffusion regions 924 with highersurface phosphorus concentrations on the top silicon hexagonal ridgesdirectly in contact with and underneath the cured n-type dopant solidsource layer 922. Through gas or vapor-phase transport of the vaporizeddopant source to the adjacent frontside regions within thehexagonal-pyramid cavities, the furnace anneal/diffusion processconcurrently dopes the remaining frontside surface regions 926 notcovered with the solid dopant source layer with phosphorus with smallersurface concentration and smaller dose, thus, creating self-alignedselective emitter regions with lighter surface doping. These lessheavily doped regions 926 improve the blue response of the solar cell,while the more heavily doped ridges 924 will minimize the frontsidecontact resistance of the solar cell for improved cell emitter contactmetallization. Similarly, the same furnace anneal process produces moreheavily doped p⁺⁻ doped 928 hexagonal-pyramid rear tips 434 for low basecontact resistance, while the remaining backside base regions 930 areless heavily doped on the surface. In the embodiment shown in FIG. 70, L392 is much less than H 462 and much less than h 440, with H 462 between100 and 500 microns, while L 932 is between 2 and 20 microns. Also shownare l 934 and α 148, where l=L/cos(α/2) and α/2=tan⁻¹(h/2H). Also shownare L′ 936, and l′ 938, where l′ is the slanted height of heavily dopedjunction region.

FIGS. 71 and 72 show YY cross-sectional views 940 and 950 afterself-aligned formation of the emitter and base contacts and solar cellinterconnects. As shown, the emitter contact is preferably wrappedaround (or it may be wrapped-through frame holes not shown) the 3-Dsubstrate frame to make all the cell contacts on the rear side of thesolar cell (for ease of solar module assembly automation). As shown inFIG. 71, the self-aligned frontside honeycomb contacts 312 are placedwithin the more heavily n⁺⁻doped top ridges 924 of the honeycombstructure (thus, producing very low contact resistance). The remainingtop regions not covered by the emitter contacts (which is most of thehexagonal-pyramid sidewall area doped by proximity vapor-phase dopingfrom the adjacent solid dopant source regions) has the less heavilydoped n⁺ selective emitter regions, enabling excellent solar cell blueresponse. The base metallization contacts 314 on the rear sidehexagonal-pyramid tips cover the more heavily p⁺⁻doped regions (formedby direct contact with the solid dopant source layer), resulting in lowbase contact metallization resistance. The remaining base surfaceregions on the rear side of the 3-D substrate are less heavily dopedwith boron (by proximity vapor-phase doping from the adjacent soliddopant source regions), enabling very low surface recombination velocityand improved cell performance. Also shown is surface passivation & ARClayers (thermal SiO₂ and PECVD or PVD hydrogenated SiN_(x)) 942. FIG. 72shows a view 950 of multiple unit cells 116, with self-alignedperipheral emitter wrap-around contact 952 connected to the solar cellhexagonal frontside emitter contact at the frame edge. Note that bothfrontside 312 and backside contacts 314 are accessible on the rear sideof the cell for ease of automated module assembly.

FIGS. 73 and 74 show YY cross-sectional views 960 and 970 of the 3-Dhexagonal-pyramid solar cells (showing a single hexagonal-pyramid unitcell and several adjacent unit cells, respectively) after completion ofthe solar cell fabrication process and after soldering (or connectingwith a suitable electrically conductive epoxy) 962 the rear basecontacts 314 (rear hexagonal-pyramid metallized tips) to the rear cellmirror 964 (and base interconnect) plate. This mirror/interconnect platemay be made of a number of materials, preferably Ag-coated Cu orAg-coated Al (or any other suitable electrically conductive andoptically reflecting material). The combination of the highly reflectiverear mirror and the 3-D hexagonal-pyramid structure of the solar cell(and the frontside honeycomb pattern aperture 972 of the solar cell)ensure extremely efficient light trapping, enablingultra-high-efficiency solar cells using very thin crystalline siliconfilms.

The following FIGURES show alternative views, corresponding to varioussteps in the process flows for fabrication of TFSCs outlined above.

FIG. 75 shows a YY cross-sectional view 980 of the 3-D hexagonal-pyramidTFSC substrate 112 (showing one pyramid unit cell) after self-alignedroller coating (or spray coating or liquid-dip coating or anothersuitable liquid-transfer coating) of n-type dopant paste/liquid 922 onthe frontside honeycomb ridges 492, and after drying/curing and furnaceannealing, preferably in an in-line belt furnace, to form the selectiveemitter regions and heavily-doped emitter contact diffusion regions (thepreferred embodiment applies both n-type and p-type dopant pastes orliquids on the frontside and backside, respectively, before a singlefurnace anneal/diffusion process to form the doped diffusion regions,including selective emitter junction regions). This structure shows theunit cell after short thermal oxidation (e.g., to grow 5 nm to 100 nmthermal oxide) and PVD or PECVD ARC layer (SiN_(x)) formation. Thedotted lines show the doped regions (after further processing, the unitcell structure is shown in FIG. 70). This embodiment shows no prior p⁺base contact doping (it will be done by Al doping in conjunction with Alrear base contact formation using a base contact firing process).

FIG. 76 is similar to FIG. 75, except FIG. 76 shows p⁺ base contactdoping by roller coating (or spray coating or liquid-dip coating oranother suitable liquid-transfer coating) of a p-type dopant layer andsubsequent curing and anneal (same anneal as emitter). Self-alignedsolid-dopant-source-doped rear p⁺ contact regions 982 (for plated rearbase contacts) are illustrated using dotted lines.

FIG. 77 shows a cross-sectional view 790 similar to the view 920 shownin FIG. 70. However, in the embodiment shown in FIG. 77 there is onlycoating of n-type dopant paste/liquid on the frontside honeycomb ridges.There is no p₊ dopant paste/liquid applied to the backside.

FIG. 78 shows a YY cross-sectional view 1010 of a 3-D hexagonal-pyramidsubstrate 112 after self-aligned formation of the frontside (top) solidn⁺ (e.g., doped with phosphorus) solid dopant source layer and selectiveemitter. This structure leads to the structure shown in FIG. 72. Thefrontside pattern consists of honeycomb ridges which will besubsequently used for formation of the emitter contacts/interconnects.FIG. 79 shows a ZZ cross-sectional view 1020.

FIG. 80 shows a YY cross-sectional view 1030 of a 3-D hexagonal-pyramidsubstrate 112 after self-aligned formation of the frontside (top) solidn⁺ (e.g., doped with phosphorus) and p⁺ solid dopant source layer,selective emitter, as well as the self-aligned frontside emitter andrear base contacts (shown with fire-through metallization but selectiveplating may also be used to obtain the same structure). This structureleads to the structure shown in FIG. 72. The frontside pattern consistsof honeycomb ridges which will be subsequently used for formation of theemitter contacts/interconnects. FIG. 81 shows a ZZ cross-sectional view1040.

FIG. 82 shows a view 1050 of a 3-D self-supporting hexagonal-pyramidTFSC substrate with thick silicon frame, compared to the thin frameshown in FIG. 72. The thick frame may have a width of roughly 500 to1000 microns and may be fused to a 3-D TFSC substrate 112 during theepitaxial silicon deposition process (by placing the thick silicon frameon the reusable silicon template) or after silicon epitaxy and 3-Dsubstrate release (by e-beam welding).

FIG. 83 shows a schematic quasi-3-D view 1060 of a hexagonal-pyramidunit cell of one embodiment of a 3-D TFSC substrate structure of thedisclosed subject matter after formation of the self-aligned frontsideemitter contact (on the honeycomb ridges) and the rear base contact (onthe hexagonal-pyramid rear tips). Solar light enter the solar cell fromthe topside into the hexagonal-pyramid unit cell cavities.

Finally, FIG. 84 shows a view 1070 an alternative embodiment of solarcell assembly on the rear mirror and base interconnect (the firstembodiment shown in FIG. 74). In contrast to FIG. 74 shown with aspecular rear mirror, this embodiment uses a diffused rear base mirror1072 (with a roughened Ag-coated surface to scatter the reflected lightback into the 3-D cell structure).

In summary, the disclosed subject matter provides solar modulestructures and methods for assembling solar module structures. The solarmodule structures comprise pyramidal three-dimensional thin-film solarcells arranged in solar module structures. The pyramidalthree-dimensional thin-film solar cell comprises a semiconductorsubstrate with self-aligned selective emitter regions and self-alignedbase diffusion regions.

The foregoing description of the preferred embodiments is provided toenable any person skilled in the art to make or use the claimed subjectmatter. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinnovative faculty. Thus, the claimed subject matter is not intended tobe limited to the embodiments shown herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A solar module structure, comprising: a topencapsulant layer; a plurality of pyramidal three-dimensional thin-filmsolar cells, each thin-film solar cell comprising: a pyramidalthree-dimensional thin-film solar cell substrate comprising emitterjunction regions and doped base regions; emitter metallization regions;and base metallization regions; wherein said pyramidal three-dimensionalthin-film solar cell substrate comprises a plurality of pyramid-shapedunit cells; a printed circuit board; a rear encapsulant layer; and aprotective back plate.
 2. The solar module structure of claim 1, furthercomprising a glass layer.
 3. The solar module structure of claim 2,wherein said glass layer comprises a separate glass cover.
 4. The solarmodule structure of claim 2, wherein said glass layer comprises aglass-like protective coating formed by a coating method.
 5. The solarmodule structure of claim 1, further comprising a top patterned cellinterconnect layer.
 6. The solar module structure of claim 1, furthercomprising a rear patterned cell interconnect layer.
 7. The solar modulestructure of claim 1, wherein said protective back plate comprises aTedlar protective back plate.
 8. The solar module structure of claim 1,wherein each pyramidal three-dimensional thin-film solar cell furthercomprises a rear mirror.
 9. The solar module structure of claim 8,wherein said rear mirror comprises an integrated rear mirror.
 10. Thesolar module structure of claim 8, wherein said rear mirror comprises adetached rear mirror.
 11. The solar module structure of claim 1, whereineach pyramidal three-dimensional thin-film solar cell further comprisesa peripheral cell frame.
 12. The solar module structure of claim 11,wherein said peripheral cell frame may be used to produce wrap-throughor wrap-around emitter metallization for making contacts to the cellemitter at the bottom of each three-dimensional thin-film solar cell.13. A method for assembling a solar module structure, comprising:patterning a frontside and a backside of a double-sided printed circuitboard coated with metallic foils according to desired frontside andbackside interconnect layouts; applying a first coating layer to therear side of a plurality of pyramidal three-dimensional thin-film solarcells, each pyramidal three-dimensional thin-film solar cell comprising:a pyramidal three-dimensional thin-film solar cell substrate comprisingemitter junction regions and doped base regions; emitter metallizationregions; and base metallization regions; wherein said pyramidalthree-dimensional thin-film solar cell substrate comprises a pluralityof pyramid-shaped unit cells; placing said plurality of pyramidalthree-dimensional thin-film solar cells on the frontside of saiddouble-sided printed circuit board; preparing a solar module assembly,comprising: a glass layer; a top encapsulant layer; said plurality ofpyramidal three-dimensional thin-film solar cells on the frontside ofsaid double-sided printed circuit board; a rear encapsulant layer; aprotective back plate; and sealing and packaging said solar moduleassembly.
 14. The method for assembling a solar module structure ofclaim 13, wherein each pyramidal three-dimensional thin-film solar cellfurther comprises a rear mirror.
 15. The method for assembling a solarmodule structure of claim 13, further comprising arranging a pluralityof said solar module assemblies in series.
 16. The method for assemblinga solar module structure of claim 13, further comprising arranging aplurality of said solar module assemblies in parallel.
 17. The methodfor assembling a solar module structure of claim 15, further comprisingconnecting at least one pair of module power output electrical leads.18. The method for assembling a solar module structure of claim 16,further comprising connecting at least one pair of module power outputelectrical leads.
 19. A solar module structure, comprising: a top glassplate; a plurality of pyramidal three-dimensional thin-film solar cells,each three-dimensional thin-film solar cell comprising: a pyramidalthree-dimensional thin-film solar cell substrate comprising emitterjunction regions and doped base regions; emitter metallization regions;and base metallization regions; wherein said pyramidal three-dimensionalthin-film solar cell substrate comprises a plurality of pyramid-shapedunit cells; a rear patterned cell interconnect layer; and a bottom glassplate.
 20. The solar module structure of claim 19, wherein eachpyramidal three-dimensional thin-film solar cell further comprises aperipheral cell frame.